232 articles
Cobaltite
Cobaltite is a silver-white cobalt sulfarsenide mineral, CoAsS, with a pyrite-related structure, found in high-temperature hydrothermal veins and mined as an industrial cobalt ore, notably at Bou Azzer, Morocco and Blackbird, Idaho.
Coesite
Coesite is a dense, high-pressure polymorph of silicon dioxide (SiO₂), synthesized by Loring Coes in 1953 and first found in nature at Meteor Crater, Arizona, in 1960.
Coffinite
Coffinite is a uranium-bearing silicate mineral with formula USiO₄, first described in 1955 from Colorado Plateau deposits, and the second most abundant uranium mineral after uraninite.
Colemanite
Colemanite is a hydrous calcium borate mineral and a principal ore of boron, first found in 1882 in Death Valley and now mined chiefly in Turkey and California.
Colusite
Colusite is a vanadium-bearing copper sulfide mineral, Cu26V2(As,Sb,Sn,Ge)6S32, whose synthetic analogs with formula Cu26T2M6S32 are earth-abundant p-type thermoelectric materials reaching ZT near 1.0 at about 700 K.
Cordierite
Cordierite is a magnesium aluminum cyclosilicate mineral that forms in thermally metamorphosed rocks; its transparent violet-blue gem variety, iolite, is also used synthetically in catalytic converter substrates.
Corundum
Corundum is a crystalline form of aluminum oxide (Al₂O₃) and the mineral that, with trace coloring elements, produces ruby and sapphire; it ranks 9 on the Mohs scale.
Covellite
Covellite is a copper sulfide mineral, CuS, known for its deep indigo-blue iridescent color; named in 1832 for Niccolò Covelli, who discovered it at Mount Vesuvius.
Cristobalite
Cristobalite is a crystalline polymorph of silica, SiO2, thermodynamically stable only above 1470 °C but common metastably in volcanic domes, opal, and even lunar rock, and a regulated cause of silicosis.
Cryptomelane
Cryptomelane is a potassium manganese oxide mineral, the potassium-rich end-member of the hollandite group, with 2 × 2 tunnels hosting K⁺; it dates supergene weathering and serves as the catalyst K-OMS-2.
Cullinan Diamond
The Cullinan Diamond, a 3,106-carat stone, was the largest gem-quality rough diamond ever found, recovered at the Premier Mine near Pretoria, South Africa in 1905.
Cummingtonite
Cummingtonite is a metamorphic rock-forming amphibole mineral, a magnesium iron silicate named in 1824 for Cummington, Massachusetts, that occurs chiefly in metamorphosed iron formations and forms a series with grunerite.
Delafossite
Delafossite is a copper iron oxide mineral, CuFeO₂, first described in 1873 from Russia and named for Gabriel Delafosse; it typifies the layered delafossite oxide family.
Diamond
Diamond is a mineral of crystallized elemental carbon, prized as a gemstone and used industrially; it forms deep in the mantle and is the hardest known natural material.
Diamond (gemstone)
Diamond is a gemstone cut from the mineral diamond, a pure form of carbon that is the hardest known natural material and the world's most popular gemstone.
Diamond clarity
Diamond clarity is one of the 4Cs of diamond grading, measuring the absence of internal inclusions and surface blemishes on a GIA scale from Flawless to I3.
Diamond color
Diamond color is the gem-trade grade of how little yellow or brown tint a diamond shows, graded D (colorless) to Z on the GIA scale.
Diaspore
Diaspore is an aluminum oxide hydroxide mineral, a principal aluminum-bearing component of bauxite alongside gibbsite and böhmite, and a color-change gemstone sold as Zultanite or Csarite.
Dickite
Dickite is a clay mineral of the kaolinite group, one of three natural polytypes alongside kaolinite and nacrite, named for Allan B. Dick, who described Welsh material in 1888.
Digenite
Digenite is a copper sulfide mineral with formula Cu₉S₅ (Cu₁.₈S), an opaque blue to black ore mineral in primary and secondary copper deposits, named in 1844 and found at Sangerhausen, Germany, and Butte, Montana.
Diopside
Diopside is a calcium magnesium silicate pyroxene, MgCaSi₂O₆, common in olivine-rich basalts and metamorphosed limestones worldwide, cut into vivid green gemstones, and used to study Earth's mantle formation conditions.
Dioptase
Dioptase is a hydrated copper cyclosilicate mineral with intense emerald-green to blue-green crystals, named by René Just Haüy in 1797 and prized as a collector's specimen.
Dolomite (mineral)
Dolomite, also called pearl spar, is a calcium magnesium carbonate mineral with formula CaMg(CO3)2, one of the three most abundant carbonate minerals alongside calcite and aragonite.
Dravite
Dravite is a magnesium-rich member of the tourmaline group of minerals, named in the 1880s for the Drava river near its type locality in Slovenia.
Dualite
Dualite is a rare yellow zircono-titanosilicate mineral of the eudialyte group, approved by the International Mineralogical Association in 2005 and found at Mount Alluaiv, Russia.
Emerald
Emerald is the green gem variety of the mineral beryl, colored by trace chromium or vanadium; the most prized stones come from Colombia, with Zambia, Brazil, and Zimbabwe also major sources.
Enstatite
Enstatite is a rock-forming silicate mineral of the pyroxene group with ideal formula Mg2Si2O6, occurring chiefly in ultramafic igneous rocks and dominant in the enstatite chondrite meteorites.
Fayalite
Fayalite is the iron-rich end-member of the olivine group of nesosilicate minerals, with formula Fe₂SiO₄, forming a solid-solution series with forsterite that defines natural olivine.
Feldspar
Feldspar is a group of rock-forming aluminosilicate minerals containing potassium, sodium, or calcium; it makes up about 60 percent of the Earth's crust and is used mainly in glassmaking and ceramics.
Ferrihydrite
Ferrihydrite is a widespread, poorly crystalline iron(III) oxyhydroxide nanomineral, named in 1971, that forms rust-colored precipitates in soils, springs, and acid mine drainage, and has been identified on Mars.