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Cuprite from Morenci, Arizona
|Crystal class||Hexoctahedral (m3m) |
H-M symbol: (4/m 3 2/m)
|Unit cell||a = 4.2685 Å; V = 77.77 Å3; Z = 2|
|Color||Dark red to conchineal red, sometimes almost black|
|Crystal habit||Cubic, octahedral, and dodecahedral crystals; as hairlike capillary forms, earthy, compact granular and massive|
|Cleavage||Fair in four directions forming octahedrons|
|Fracture||Conchoidal to uneven|
|Mohs scale hardness||3.5 to 4|
|Luster||Adamantine, sub-metallic, earthy|
|Streak||Shining metallic brownish-red|
|Refractive index||n = 2.849|
Its dark crystals with red internal reflections are in the isometric system hexoctahedral class, appearing as cubic, octahedral, or dodecahedral forms, or in combinations. Penetration twins frequently occur. In spite of its nice color it is rarely used for jewelry because of its low Mohs hardness of 3.5 to 4. It has a relatively high specific gravity of 6.1, imperfect cleavage and is brittle to conchoidal fracture. The luster is sub-metallic to brilliant adamantine. The "chalcotrichite" (from Ancient Greek: χαλκός θρίξ τριχός, "plush copper ore") variety typically shows greatly elongated (parallel to ) capillary or needle like crystals forms.
It is a secondary mineral which forms in the oxidized zone of copper sulfide deposits. It frequently occurs in association with native copper, azurite, chrysocolla, malachite, tenorite and a variety of iron oxide minerals. It is known as ruby copper due to its distinctive red color.
Though almost all crystals of cuprite are far too small to yield faceted gemstones, one unique deposit from Onganja in Seeis, Namibia, which was discovered in the 1970s, has produced crystals which were both large and gem quality. Virtually every faceted stone over one carat (0.2 gm) in weight is from this single deposit, which has long since been mined out. The number of faceted gems over two carats (0.4 gm) is difficult to estimate, but according to Joel Arem, one-time curator for the Smithsonian National Gem and Mineral Collection in Washington DC, faceted cuprite of any size is considered one of the most collectible and spectacular gems in existence, with its deep garnet coloring and higher brilliance than a diamond. Only the gem's soft nature prevents it from being among the most valuable jewelry stones.