Metamorphic Rocks

1. Anthracite

Anthracite is a type of coal that is black, shiny, hard and has a high carbon content. It is the highest rank of coal, meaning it has undergone the longest and most intense period of pressure and heat to form. As a result, it has a very low moisture content and a high energy content.

Anthracite is primarily used as a fuel for heating and in industrial processes where high temperatures are required. It is also used in the production of steel and as a reducing agent in the manufacturing of various chemicals. Because of its high energy content and low pollution, it is considered a clean-burning fuel compared to other types of coal.

Anthracite is found in large deposits in the eastern United States, primarily in Pennsylvania, as well as in other parts of the world, including China, Russia, and South Africa.

2. Amphibolite

Amphibolite is a metamorphic rock composed mainly of amphibole and plagioclase feldspar. It typically forms from basaltic rocks that have undergone metamorphism, with the mineral composition and texture of the original rock being altered due to heat and pressure. Amphibolite is usually dark green or black in colour, with a granular or foliated texture, and is commonly used in construction as a building material or decorative stone. It can also be used as a source of industrial minerals such as asbestos or talc. Amphibolite is an important rock type in the geology of mountain ranges and is commonly found in regions of high metamorphic activity, such as the Himalayas or the Alps.

3. Blueschist

Blueschist is a type of metamorphic rock that forms under high-pressure, low-temperature conditions. It typically has a blue-grey colour and a fine-grained texture. Blueschist is often found in subduction zones, where one tectonic plate is forced beneath another. The high-pressure and low-temperature conditions in these zones cause the minerals in the rock to re-crystallize, forming blueschist. Some common minerals found in blueschist include glaucophane, lawsonite, and garnet. Blueschist is an important rock type for geologists because it can provide insight into the processes that occur at subduction zones.

4. Cataclasite

Cataclasite is a type of rock formed by the grinding and crushing of pre-existing rocks along a fault line during tectonic activity. It is essentially a type of fault breccia, consisting of a mix of rock fragments of various sizes, held together by a matrix of crushed and pulverized rock. Cataclasite typically forms in shallow crustal levels, at depths of a few kilometres, and can be found in areas with high levels of tectonic activity, such as along active faults, shear zones, and thrust faults. The name cataclastic comes from the Greek word “kataklasis,” meaning “crushing” or “breaking.”

5. Eclogite

Eclogite is a type of metamorphic rock that is formed under high pressure and temperature conditions in the Earth’s crust. It is typically composed of two main minerals: garnet and omphacite, which are both indicative of high-pressure conditions. Eclogite is typically found in areas where subduction zones have occurred, such as mountain belts and ancient oceanic crust.

The unique composition of eclogite makes it a valuable indicator of the geologic processes that have shaped the Earth’s surface over millions of years. For example, the presence of eclogite in certain areas can help geologists identify areas where subduction has occurred and can provide clues about the composition and history of ancient oceanic crust. Additionally, the presence of eclogite can provide insight into the deep-seated processes that drive plate tectonics and the formation of mountains.

6. Gneiss

Gneiss is a metamorphic rock that is formed through the process of high pressure and temperature. It is made up of minerals such as quartz, feldspar, mica, and hornblende, which are layered and folded to create a distinct banding pattern. Gneiss can vary in colour from light to dark and can also be found in various textures depending on its formation. It is commonly used in construction and as a decorative stone due to its durability and attractive appearance.

7. Granulite

Granulite is a metamorphic rock that is composed of granular minerals, typically composed of feldspar, quartz, and mica. It is formed by the high-grade metamorphism of sedimentary or igneous rocks, typically at depths of 10-15 km and temperatures between 600-800°C. The term granulite is used to describe a range of metamorphic rocks that are characterized by a granular texture and a mineral assemblage that is typical of high-grade metamorphism. Granulite is often used as a building material and is also used in the production of ceramic tiles and other decorative objects.

8. Greenschist

Greenschist is a type of metamorphic rock that forms from the alteration of basalt or other volcanic rocks. It is characterized by its green colour, which is caused by the presence of chlorite and other green minerals such as epidote, actinolite, and albite. Greenschist typically has a fine-grained texture and is often used as a decorative stone or building material. It is commonly found in regions where there has been significant tectonic activity, such as along the edges of continents or in areas of mountain building.

9. Hornfels

Hornfels is a hard, fine-grained metamorphic rock that forms through the process of contact metamorphism. It is typically formed from pre-existing rocks, such as shale or sandstone, that are subjected to high temperatures and pressures from the intrusion of igneous rocks, such as granite or basalt.

During this process, the minerals in the original rock recrystallize and fuse together, creating a dense and durable rock with a characteristic dark, glossy appearance. Hornfels is often found in areas where contact metamorphism has occurred, such as near the margins of intrusive igneous rocks or along fault zones.

Hornfels is used as a construction material, as well as in the production of industrial minerals and decorative stones. It is also studied by geologists as an important indicator of past tectonic activity and geological history.

10. Calcflinta

Calcflinta, or calc-flinta, is a fine-grained calc–silicate rock found amongst the metamorphic rocks of the eastern Highlands of Scotland. It is a hornfels developed from calcareous mudstone. Calcflinta is also found, for example, around the northwest margin of the Dartmoor granite in England, and on King Island in Tasmania.

11. Litchfieldite

Litchfieldite is a rare mineral that is a variety of uraninite, a uranium oxide mineral. It was first discovered in 1931 at the Litchfield Mine in Ontario, Canada. Litchfieldite is typically black or brownish-black in colour, with a metallic lustre and a high density. It is found in granitic pegmatites and hydrothermal veins associated with uranium deposits. Litchfieldite has radioactive properties due to its uranium content and should be handled with care. It is of interest to mineral collectors and uranium ore prospectors but has no commercial value.

12. Marble

Marble is a metamorphic rock that is formed from the recrystallization of limestone under high pressure and temperature. It is composed primarily of calcium carbonate and can be found in a range of colours, from white to black, with veins of various shades running through it. Marble has been used for centuries as a building material, decorative element, and for sculpture due to its durability, beauty, and ease with which it can be polished. Some famous structures made from marble include the Taj Mahal, the Lincoln Memorial, and the Parthenon.

13. Migmatite

Migmatite is a type of rock that is a mixture of metamorphic and igneous rocks. It is formed when a pre-existing rock, usually a metamorphic rock, is partially melted due to high heat and pressure, and then recrystallizes. Migmatites often have a banded or layered appearance, with lighter-coloured bands of felsic igneous rock (such as granite) intermingled with darker-coloured bands of metamorphic rock (such as gneiss). They can be found in areas of high-grade metamorphisms, such as mountain belts or deep within the Earth’s crust. Migmatites are important in understanding the processes of mountain building and the evolution of the Earth’s crust.

14. Mylonite

Mylonite is a type of metamorphic rock that forms through the deformation and shearing of pre-existing rocks. It is a fine-grained rock that typically has a strong foliation or banding, indicating the direction of deformation. Mylonites can form from a variety of rock types, including igneous, sedimentary, and other metamorphic rocks.

The formation of mylonite typically involves high temperatures and pressures, along with significant mechanical deformation. This can occur during mountain-building events such as plate collisions or during the shearing of rocks along fault zones. The resulting mylonite is typically harder and denser than the original rock and may contain elongated mineral grains or clasts that align with the direction of deformation.

Mylonite has a variety of uses, including as a decorative stone, in construction and paving, and in the manufacture of abrasives and polishing compounds. It is also of interest to geologists and structural geologists for its insights into tectonic processes and deformation mechanisms.

15. Metapelite

Metapelite is a type of metamorphic rock that forms from the alteration of shale, mudstone, or other clay-rich sedimentary rocks. The term “metapelite” is derived from the Greek words “meta” meaning change and “pelite” meaning mudstone or shale. Metapelite typically forms under conditions of high pressure and temperature, which causes the clay minerals in the original sedimentary rock to recrystallize and form new minerals such as mica, chlorite, and garnet. Metapelite is commonly found in regions that have undergone intense tectonic activity, such as mountain ranges or areas of continental collision. It is often used as an indicator of the tectonic history of a particular region, as the mineral composition and texture of the rock can provide insights into the pressure, temperature, and duration of the metamorphic event.

16. Metapsammite

Metapsammite is a type of rock that is formed by the transformation of sandstone under high pressure and temperature. It is a metamorphic rock that has a fine-grained texture and is composed primarily of quartz and feldspar, along with small amounts of other minerals such as mica and chlorite. Metapsammite is often found in regions that have undergone tectonic activity and metamorphism, such as mountain ranges and regions near plate boundaries. It is commonly used in construction and as a decorative stone due to its durability and attractive appearance.

17. Phyllite

Phyllite is a type of metamorphic rock that is formed from shale or mudstone through low-grade regional metamorphism. It has a fine-grained texture and a shiny, slaty appearance due to the alignment of its mineral grains. The minerals that makeup phyllite are typically muscovite, biotite, chlorite, and quartz. Phyllite is often used as a decorative stone for building facades and interior flooring due to its attractive appearance and durability. It is also used in geology for studying the processes of regional metamorphism.

18. Pseudotachylite

Pseudotachylite is a type of fine-grained rock that is formed by the melting and rapid cooling of rock during an earthquake. It is often found along fault zones and is characterized by its dark, glassy appearance. The name “pseudotachylite” comes from the Greek word “pseudo,” meaning false, and the word “tachylite,” which refers to a type of volcanic glass.

Pseudotachylite is often found in areas where there has been significant tectonic activity, such as the boundary between two tectonic plates or along a fault zone. It is formed when the intense heat and pressure generated during an earthquake cause rocks to melt and then rapidly cool, forming a glassy texture.

Scientists study pseudotachylite to better understand the mechanics of earthquakes and fault zones. By analyzing the composition and structure of these rocks, they can learn more about the forces involved in seismic activity and the processes that shape the earth’s surface.

19. Quartzite

Quartzite is a metamorphic rock that is formed from sandstone under high pressure and temperature. It is composed of pure quartz grains that have been fused together, resulting in a very hard and durable material that is resistant to weathering and erosion. The colour of quartzite ranges from white, grey, yellow, pink, green, and red, depending on the mineral content and impurities present in the rock. Due to its durability and natural beauty, quartzite is commonly used in construction as a decorative and structural material, such as for flooring, wall cladding, countertops, and building facades. It is also used in the manufacturing of glass and ceramics, and as a raw material for the production of silicon metal and ferrosilicon alloys.

20. Schist

Schist is a metamorphic rock that is characterized by its coarse-grained texture and foliated appearance. It is formed through the process of regional metamorphism, which involves intense heat and pressure applied to existing rocks over time. Schist is made up of minerals such as mica, quartz, and feldspar, and is commonly used in construction and landscaping due to its durability and attractive appearance. Schist can also be used in the production of crushed stone and as a decorative stone in gardens and outdoor spaces.

21. Serpentinite

Serpentinite is a type of rock that forms when peridotite, a rock that makes up the Earth’s upper mantle, is altered by water and other fluids. The process of alteration is called serpentinization, which involves the breakdown of minerals in peridotite and the formation of new ones. Serpentinite is typically green in color and has a smooth, polished appearance. It is commonly found in areas where there has been extensive tectonic activity, such as along fault zones or at the edges of tectonic plates. Serpentinite is important in geology because it can serve as a source of magnesium, a mineral that is used in many industrial processes. Additionally, serpentinite can act as a host rock for certain types of minerals and ores, including asbestos, talc, and chromite.

22. Skarn

Skarn is a type of rock formation that is typically found in contact zones between igneous and sedimentary rocks. Skarns are formed when hot fluids from magmatic intrusions or hydrothermal systems interact with surrounding rocks, causing mineral alteration and the formation of new mineral assemblages.

Skarns can contain a variety of minerals, including garnet, pyroxene, wollastonite, and other silicate minerals. They are often rich in valuable metals, such as copper, gold, silver, and iron, and are therefore of significant economic interest for mining.

Skarns can have a wide range of colours and textures, depending on the type and amount of minerals present. They are commonly found in mountainous regions, where they can be exposed through erosion or uplift.

23. Slate

Slate is a fine-grained metamorphic rock that is formed by the alteration of shale or mudstone through intense pressure and heat. It is composed of clay minerals, mica, quartz, and other minerals that have undergone a process of recrystallization. Slate is typically grey in colour but can also be found in various shades of green, red, purple, and black. It is a popular material for roofing, flooring, and other construction applications due to its durability, weather resistance, and natural beauty. Slate is also used for decorative purposes such as wall cladding, fireplace surrounds, and outdoor landscaping features.

24. Suevite

Suevite is a rock consisting partly of melted material, typically forming a breccia containing glass and crystal or lithic fragments, formed during an impact event. It forms part of a group of rock types and structures that are known as impactites. The word “suevite” is derived from “Suevia”, the Latin name of Swabia. It was suggested by Adolf Sauer in 1901. Suevite is thought to form in and around impact craters by the sintering of molten fragments together with unmelted clasts of the country rock. Rocks formed from more completely melted material found in the crater floor are known as tagamites. Suevite is distinct from pseudotachylite in an impact structure as the latter is thought to have formed by frictional effects within the crater floor and below the crater during the initial compression phase of the impact and the subsequent formation of the central uplift.

25. Talc carbonate

Talc carbonates are a suite of rock and mineral compositions found in metamorphosed ultramafic rocks.

The term refers to the two most common end-member minerals found within ultramafic rocks which have undergone talc-carbonation or carbonation reactions: talc and the carbonate mineral magnesite.

Talc carbonate mineral assemblages are controlled by the temperature and pressure of metamorphism and the partial pressure of carbon dioxide within metamorphic fluids, as well as by the composition of the host rock.

26. Soapstone

Soapstone also known as Steatite, is a metamorphic rock that consists primarily of talc. Depending on the quarry from which it is sourced, this natural stone also contains varying amounts of other minerals such as micas, chlorite, amphiboles, quartz, magnesite, and carbonates. It is a relatively soft, very dense, highly heat-resistant material.

27. Tectonite

Tectonites are metamorphic or tectonically deformed rocks whose fabric reflects the history of their deformation, or rocks with fabric that clearly displays coordinated geometric features that indicate continuous solid (ductile) flow during formation. Planar foliation results from a parallel orientation of platey mineral phases such as phyllosilicates or graphite. Slender prismatic crystals such as amphibole produce a lineation in which these prisms or columnar crystals become aligned. Tectonites are rocks with minerals that have been affected by the natural forces of the earth, which allowed their orientations to change. This usually includes the recrystallization of minerals and foliation formation.

28. Whiteschist

A whiteschist is an uncommon metamorphic rock formed at high to ultra-high pressures. It has the characteristic mineral assemblage of kyanite + talc, responsible for its white colour. The name was introduced in 1973 by German mineralogist and petrologist Werner Schreyer. This rock is associated with the metamorphism of some pelites, evaporite sequences or altered basaltic or felsic intrusions. Whiteschists form in the MgO–Fe₂O₃–Al₂O₃–SiO₂–H₂O (MFASH) system. Rocks of this primary chemistry are extremely uncommon and they are in most cases thought to be the result of metasomatic alteration, with the removal of various mobile elements.

Specific varieties

The following are terms for rocks that are not petrographically or genetically distinct but are defined according to various other criteria; most are specific classes of other rocks or altered versions of existing rocks. Some archaic and vernacular terms for rocks are also included.

1. Appinite

Appinite rock is a type of metamorphic rock that is characterized by a green colour and a wavy or layered texture. It is formed from the alteration of volcanic rocks under high temperature and pressure conditions. Appinite rocks are typically composed of minerals such as actinolite, chlorite, epidote, and calcite. They are commonly found in regions that have undergone intense tectonic activity, such as mountain ranges and subduction zones. Appinite rock is often used in construction as a decorative stone or as a source of crushed stone for roads and other infrastructure projects.

2. Borolanite

Borolanite is a rare semi-metamorphic igneous rock that is composed of feldspar, mafic minerals, and minor amounts of quartz. It is commonly found in Scotland and is named after the Borralan area in Sutherland.

Borolanite rock is mainly used as a decorative stone in landscaping and construction. It is known for its attractive dark colour, uniform texture, and durability. The rock is also used in the construction of memorials, monuments, and buildings.

In terms of geology, Borolanite rock is formed from the crystallization of magma deep beneath the Earth’s crust. The cooling and solidification of the magma result in the formation of mineral crystals, giving the rock its distinct appearance and properties.

Overall, Borolanite rock is a unique and valuable resource that has a variety of uses in the construction and decorative industries.

3. Blue Granite

Blue granite is a type of igneous rock that is known for its unique blue color. It is formed from the slow cooling and solidification of magma or lava deep beneath the earth’s surface. Blue granite is composed mainly of feldspar, quartz, and mica, with other minerals present in smaller amounts. It is a durable and popular choice for countertops, flooring, and other building materials due to its strength and natural beauty. Some popular types of blue granite include Blue Bahia, Blue Pearl, and Labradorite Blue.

4. Epidosite

Epidosite is a type of metamorphic rock that is characterized by the presence of epidote, a green-coloured mineral. The rock typically forms from the alteration of mafic or ultramafic rocks, such as basalt or gabbro, by hydrothermal fluids. The alteration process involves the replacement of minerals in the original rock with new minerals, including epidote, chlorite, quartz, and albite. The resulting rock is typically fine-grained and has a greenish colour due to the abundance of epidote. Epidosite is commonly found in areas where there has been significant hydrothermal activity, such as near volcanic or tectonic regions. It is often used as a decorative stone due to its unique colour and pattern.

5. Felsite

Felsite is a very fine-grained volcanic rock that may or may not contain larger crystals. Felsite is a field term for a light-coloured rock that typically requires a petrographic examination or chemical analysis for a more precise definition. Colour is generally white through light grey, or red to tan and may include any colour except dark gray, green or black (the colours of trap rock). The mass of the rock consists of a fine-grained matrix of felsic materials, particularly quartz, sodium and potassium feldspar,^[2] and may be termed a quartz felsite or quartz porphyry if the quartz phenocrysts are present. This rock is typical of extrusive origin, formed by the compaction of fine volcanic ash, and may be found in association with obsidian and rhyolite. In some cases, it is sufficiently fine-grained for use in making stone tools. Its fine texture and felsic components allow for good knapped pieces, much like working chert, producing conchoidal fracture.

Dendritic manganese oxides such as pyrolusite and/or iron oxides such as limonite may precipitate along rock crevices, giving some rock chunk surfaces multicoloured or arborescent patterned textures.

6. Ganister

A ganister (or sometimes gannister) is hard, fine-grained quartzose sandstone, or orthoquartzite, used in the manufacture of silica brick typically used to line furnaces. Ganisters are cemented with secondary silica and typically have a characteristic splintery fracture.

Cornish miners originally coined this term for hard, chemically and physically inert silica-cemented quartzose sandstones, commonly, but not always found as seatearths within English Carboniferous coal measures. This term is now used for similar quartzose sandstones found typically as seatearths in the Carboniferous coal measures of Nova Scotia, the United States, and the Triassic coal-bearing strata of the Sydney Basin in Australia.

Where a ganister underlies coal as a seatearth, it typically is penetrated by numerous root traces. These root traces typically consist of carbonaceous material. Ganisters that contain an abundance of fossil roots, which appear as fine carbonaceous, pencil-like streaks or markings, are called “pencil ganisters”. In other cases, the root traces consist of fine, branching nodules, called “rhizoliths”, which formed around the roots before they decayed.

From detailed studies of ganisters, geologists have concluded that the typical ganister is the silicified surface horizon, i.e. E horizon, of a buried soil, called “paleosol”, developed in sandy sediments. These and other studies have found ganisters to contain abundant evidence of having once been the upper horizon of a soil, which has developed in loamy or sandy sediments. The evidence includes some combination of carbonized roots and rootlets, rhizoliths, illuvial clay cutans, silcrete-like silica cement, and the leaching and alteration of the sandy sediments by weathering and plants. These studies argue that the destruction of easily weathered minerals, i.e. feldspar, within the surface horizon of soil by soil-forming processes is what creates the quartz-rich nature of ganisters. The silica-cementation that creates a ganister typically results from the dissolution of plant opal within a soil profile and its redeposition as silica cement within it. The formation of ganisters has been observed within modern soils, such as in the Okavango Delta of Botswana.

7. Gossan

Gossan (eiserner hut or eisenhut) is intensely oxidized, weathered or decomposed rock, usually the upper and exposed part of an ore deposit or mineral vein. In the classic gossan or iron cap all that remains is iron oxides and quartz, often in the form of boxworks (which are quartz-lined cavities retaining the shape of the dissolved ore minerals). In other cases, quartz and iron oxides, limonite, goethite, and jarosite, exist as pseudomorphs, replacing the pyrite and primary ore minerals. Frequently, gossan appears as a red “stain” against the background rock and soil, due to the abundance of oxidized iron; the gossan may be a topographic positive area due to the abundance of erosion-resistant quartz and iron oxides. Although most gossans are red, orange, or yellow, black gossans from manganese oxides such as pyrolusite, manganite, and especially psilomelane form at the oxidized portion of manganese-rich mineral deposits.

In the 19th and 20th centuries, gossans were important guides to buried ore deposits used by prospectors in their quest for metal ores. An experienced prospector could read the clues in the structure of the gossans to determine the type of mineralization likely to be found below the iron cap.

8. Hyaloclastite

Hyaloclastite is a type of volcanic rock that forms as a result of explosive interactions between magma and water or ice. The term “hyaloclastite” comes from the Greek words “hyalos,” meaning glass, and “klastos,” meaning broken.

Hyaloclastite is characterized by its glassy texture and highly fragmented appearance, with angular or rounded fragments of various sizes mixed together. It is typically found in volcanic environments that are influenced by water, such as subglacial or submarine volcanoes.

Hyaloclastite is formed when hot magma or lava comes into contact with water or ice, causing rapid cooling and fragmentation. The resulting rock is made up of volcanic glass, crystals, and various types of sediment that have been broken apart and mixed together.

Hyaloclastite can provide important clues about the history of volcanic activity and the conditions that existed during the formation of the rock. It is also an important resource for the study of geology and volcanology, as it can provide insight into the dynamics of volcanic eruptions and their effects on the surrounding environment.

9. Jadeitite

Jadeitite is a metamorphic rock found in blueschist-grade metamorphic terranes. It is found in isolated metasomatically altered rock units within serpentinite associated with subduction zone environments. Jadeitite consists almost entirely of the pyroxene mineral jadeite and is typically mined as a source of the ornamental rock or gemstone, jade. Occurrences include Myanmar, Guatemala, Japan, Kazakhstan and in the Coast Ranges of western North America.

Over 100 axe heads made from jadeitite quarried in northern Italy in the Neolithic era have been found across the British Isles. Because of the difficulty of working this material, all the axe heads of this type found are thought to have been non-utilitarian and to have represented some form of currency or be the products of gift exchange.

10. Jasperoid

Jasperoid is a rare, peculiar type of metasomatic alteration and occurs in two main forms; sulfidic jasperoids and hematitic jasperoids. True jasperoids are different from jaspillite, which is a form of metamorphosed chemical sedimentary rock, and from jasper which is a chemical sediment.

Sulfidic jasperoids are typical examples of silica-sulfide metasomatism of dolomites, and are found in Nevada, Australia and Iran. They are hard, dense purple-black rocks with considerable content of pyrite. The bodies in Nevada are quite thin (seldom greater than 8 m) and stratabound.

11. Lapis lazuli

Lapis lazuli or lapis for short, is a deep-blue metamorphic rock used as a semi-precious stone that has been prized since antiquity for its intense colour.

As early as the 7th millennium BC, lapis lazuli was mined in the Sar-i Sang mines, in Shortugai, and in other mines in Badakhshan province in northeast Afghanistan.

Lapis lazuli artefacts, dated to 7570 BC, have been found at Bhirrana, which is the oldest site of Indus Valley civilisation. Lapis was highly valued by the Indus Valley Civilisation (7570–1900 BC). Lapis beads have been found at Neolithic burials in Mehrgarh, the Caucasus, and as far away as Mauritania. It was used in the funeral mask of Tutankhamun (1341–1323 BC).

By the end of the Middle Ages, lapis lazuli began to be exported to Europe, where it was ground into powder and made into ultramarine, the finest and most expensive of all blue pigments. Ultramarine was used by some of the most important artists of the Renaissance and Baroque, including Masaccio, Perugino, Titian and Vermeer, and was often reserved for the clothing of the central figures of their paintings, especially the Virgin Mary. Ultramarine has also been found in the dental tartar of medieval nuns and scribes.

12. Litchfieldite

Litchfieldite is a rare igneous mineral rock that was first discovered in 1903 in the town of Litchfield, Maine, United States. It is a type of mineral that belongs to the tourmaline group, which is a group of complex borosilicate minerals.

Litchfieldite has a unique chemical composition compared to other tourmalines, as it contains high amounts of lithium and manganese. Its chemical formula is Li(Mn,Fe)_2Al_6(BO_3)_3Si_6O_18(OH)_4.

This mineral typically occurs in granitic pegmatites and can form as elongated prismatic crystals that are usually black or dark green in colour. Litchfieldite is not commonly used in industry, but it is highly valued by mineral collectors due to its rarity and unique chemical composition.

13. Llanite

Llanite is a porphyritic rhyolite with distinctive phenocrysts of blue quartz (a rare quartz color) and perthitic feldspar (light grayish-orangish). The brown, fine-grained groundmass consists of very small quartz, feldspar, and biotite mica crystals.

Llanite comes from a hypabyssal porphyritic rhyolite dike that intrudes Precambrian metamorphics in the Llano Uplift of central Texas. Published radiometric dating on this llanite indicates that it’s 1.106 billion years old (late Mesoproterozoic).

The quartz crystals found in llanite are blue hexagonal bipyramids. The unusual blue coloration of the quartz is thought to be due to ilmenite inclusions.

It is named after Llano County, Texas, the only place where it is found.

However, the geology of North-East Africa is very similar to that of Texas. Many of the minerals and fossils found are only found in these two locations on the whole planet. (geologists have identified other locations where similar types of rock may be found). A dike of llanite crops out on Texas State Highway 16 approximately nine miles north of the town of Llano.

Llanite, which is similar to granite, is very strong, with a crushing strength of 37,800 lb/in² or 26,577,180 kg/m². Llanite is also very similar in appearance to pietersite.

14. Luxullianite

Luxullianite is a rare granite-like rock that is primarily composed of quartz, feldspar, and blue-green amphibole. It was first discovered in the village of Luxulyan in Cornwall, England in the 19th century and was named after the village.

Luxullianite has a distinctive appearance, with a pale grey or white background colour and large, irregular-shaped blue-green crystals of the amphibole mineral riebeckite. The crystals can vary in size, from a few millimetres to several centimetres in length.

Due to its rarity and attractive appearance, luxullianite has been used as a decorative stone in buildings and monuments, particularly in Cornwall. It has also been used as a gemstone, although its use in this context is very limited due to its rarity.

Luxullianite is not widely available, and it is considered a collector’s item by many mineral enthusiasts. Its unique appearance and scarcity make it a prized addition to any collection of rocks and minerals.

15. Novaculite

It is also known as Arkansas stone. Novaculite is a fine-grained metamorphosed sedimentary rock composed primarily of silica. It is commonly found in the Ouachita Mountains of Arkansas and Oklahoma and is also present in other parts of the world such as Texas, North Carolina, and Mexico. Novaculite is known for its ability to be sharpened to a very fine edge and has been used as a whetstone for centuries. It is also used in construction and manufacturing, as well as in the production of ceramics, glass, and semiconductors. Novaculite has a distinctive white-to-grey colour and a smooth, glassy texture.

16. Pietersite

Pietersite is a rare gemstone that was discovered in Namibia in 1962. It is a type of chalcedony that is composed of different minerals, including tiger’s eye, hawk’s eye, and jasper. It is characterized by its striking combination of blue, gold, and red colours, which creates a swirling pattern that is reminiscent of a stormy sky.

Pietersite is believed to have a number of metaphysical properties, including the ability to enhance intuition and spiritual awareness, promote emotional balance and stability, and stimulate creativity and self-expression. It is also said to be a powerful stone for protection, helping to shield the wearer from negative energies and psychic attacks.

Due to its rarity and unique beauty, pietersite is highly sought after by collectors and jewellery enthusiasts. It is typically cut into cabochons or beads and used in a variety of jewellery pieces, including rings, pendants, and bracelets.

17. Pyrolite

Pyrolite is a type of rock that is a mineral assemblage of pyroxenes and plagioclase feldspars. It is mainly composed of calcium-rich pyroxene and plagioclase feldspar, with minor amounts of other minerals such as olivine and magnetite. Pyrolite is typically associated with the Earth’s upper mantle and is thought to represent the most primitive material in the mantle. It is also a major component of the Earth’s mantle and is believed to be the source of magma that rises to the surface and forms volcanic rocks. The name pyrolite is derived from the Greek words pyr, meaning fire, and lithos, meaning stone.

18. Rapakivi granite

Rapakivi granite is a type of granite that is characterized by large, round, pinkish feldspar crystals (called rapakivi) embedded in a grey or beige matrix. The name “rapakivi” comes from Finnish, meaning “crumbly rock,” because the feldspar crystals are often found in a weathered, crumbly matrix. This type of granite is found in many parts of the world but is most commonly associated with the Baltic Shield in Finland, where it is used for building and decorative purposes. The distinctive rapakivi granite is also popular for use in countertops and flooring due to its unique appearance and durability.

19. Rhomb porphyry

Rhomb porphyry is a type of volcanic rock that is characterized by its distinctive rhomb-shaped crystals of feldspar embedded in a fine-grained matrix. It is a porphyritic rock, which means that it contains large crystals (phenocrysts) embedded in a smaller-grained matrix (groundmass). The rhomb-shaped crystals of feldspar in rhomb porphyry are typically white or light grey in colour and are often surrounded by a darker-coloured matrix of pyroxene, hornblende, or biotite. This type of porphyry is formed by the slow cooling and solidification of magma deep beneath the Earth’s surface, and it is commonly used as a decorative stone in construction and landscaping.

20. Rodingite

Rodingite is a rare metamorphic (basically igneous) rock type that is composed of green amphiboles (a group of minerals) and white plagioclase feldspar. It also often contains minor amounts of other minerals such as biotite, chlorite, and garnet. Rodingite is named after the town of Roding in Germany, where it was first described. It is usually found in association with other igneous and metamorphic rocks, such as gabbro, peridotite, and serpentinite. Rodingite is known to have a significant impact on the environment, as it can contain heavy metals such as chromium and nickel that can be toxic to living organisms. It is also a popular rock type for use in building and construction due to its durability and resistance to weathering.

21. Shonkinite

Shonkinite is a rare type of volcanic rock that is found in the Shonkin Sag area of Montana, USA. It is a dark, fine-grained rock that is made up of plagioclase feldspar, augite, and olivine minerals. Shonkinite is considered to be an intrusive igneous rock, which means that it forms deep beneath the Earth’s surface and then solidifies slowly over time. It is believed that shonkinite was formed during the early stages of the formation of the Rocky Mountains, around 65 million years ago. The rock is not commonly used for construction or industrial purposes, but it is often sought after by collectors due to its unique composition and rarity.

22. Taconite

Taconite is a type of sedimentary rock that is found in the United States, specifically in Minnesota, Michigan, and Wisconsin. It is primarily made up of iron-bearing minerals, such as magnetite, hematite, and chert, and has a banded appearance due to its alternating layers of iron-rich and silica-rich material.

Taconite is important for its iron content, which can be extracted through a process called beneficiation. This involves crushing and grinding the rock, then using magnetic separation to separate the iron minerals from the silica and other impurities. The resulting iron concentrate can then be smelted to produce iron and steel.

The mining and processing of taconite have played a significant role in the economy of the Great Lakes region, providing jobs and contributing to local communities. However, it has also been associated with environmental concerns, such as water pollution and habitat destruction. Efforts are ongoing to improve the sustainability of taconite mining and processing practices.

23. Tachylite

Tachylite also known as Tachylyte is a form of basaltic volcanic glass. This glass is formed naturally by the rapid cooling of molten basalt. It is a type of mafic igneous rock that is decomposable by acids and readily fusible.^{[citation needed]} The colour is black or dark-brown, and it has a greasy-looking, resinous lustre. It is very brittle and occurs in dikes, veins, and intrusive masses. The word originates from the Ancient Greek ταχύς, meaning “swift”.

Tachylites have the appearance of pitch and are often more or less vesicular and sometimes spherulitic. They are very brittle and break down readily under a hammer. Small crystals of feldspar or olivine are sometimes visible in them with the unaided eye. All tachylites weather rather easily and by oxidation of their iron become dark brown or red. Three modes of occurrence characterize this rock. In all cases, they are found under conditions which imply rapid cooling, but they are much less common than acid volcanic glasses (or obsidians), the reason being apparent that the basic rocks have a stronger tendency to crystallize, partly because they are more liquid and the molecules have more freedom to arrange themselves in crystalline order.

Tachylite can be distinguished from obsidian and pitchstone by determining its fusibility, as splinters of tachyykite will fuse together when heated.

24. Theralite

A variety of nepheline-gabbro consisting essentially of titanian augite, labradorite and nepheline. Olivine is a variable but sometimes major constituent. May be used as a synonym for nepheline-gabbro of QAPF field 14

A group of mafic plutonic rocks composed of calcic plagioclase, feldspathoids, and augite, with lesser amounts of sodic sanidine and sodic amphiboles and accessory olivine; also, any rock in that group; the intrusive equivalent of tephrite. Theralite grades into nepheline monzonite with an increase in the alkali feldspar content, into gabbro as the feldspathoid content diminishes, and into diorite with both fewer feldspathoids and increasingly sodic plagioclase.

25. Unakite

Unakite is a metamorphic rock that is altered granite composed of pink orthoclase feldspar, green epidote, and generally colorless quartz.

It was first found in the United States in the Unaka mountain range of North Carolina from which it gets its name. Unakite exists in various shades of green and pink and is usually mottled in appearance.

A good quality unakite is considered a semiprecious stone; it will take a good polish and is often used in jewellery as beads or cabochons and other lapidary work such as eggs, spheres and animal carvings. It is also referred to as epidotized or epidote granite.

In some of the Blue Ridge occurrences, an epidotized augen gneiss is present exhibiting foliation structures.

The dominant green epidote in unakite rocks is the metasomatic alteration product of plagioclase feldspar, while the orthoclase and quartz crystals remain unaffected.

26. Vogesite

Vogesite was first described in the Vosges mountains, France, where rocks of this type (actually, minette) were described in the early 20th century.

27. Wad

Wad is an old mining term for any black manganese oxide or hydroxide mineral-rich rock in the oxidized zone of various ore deposits. Typically closely associated with various iron oxides. Specific mineral varieties include pyrolusite, lithiophorite, nsutite, takanelite and vernadite. Wad can be considered to be the manganese equivalent to the iron mineraloid limonite.