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topaz
Science; General science
A mineral best known for its use as a gemstone. Crystals are usually colorless but may be red, yellow, green, blue, or brown. The wine-yellow variety is the one usually cut and ...
diamond
Science; General science
A mineral composed entirely of carbon; the hardest substance known. Diamond is a polymorph of carbon; lonsdaleite, another polymorph, is sometimes referred to as hexagonal ...
calcite
Science; General science
A mineral composed of calcium carbonate (CaCO 3 ); one of the most common and widespread minerals in the Earth's crust. Calcite may be found in a great variety of sedimentary, ...
scheelite
Science; General science
A mineral consisting of calcium tungstate, CaWO 4 . Scheelite occurs in colorless to white, tetragonal crystals (see illus. ); it may also be massive and granular. Its fracture ...
wulfenite
Science; General science
A mineral consisting of lead molybdate, PbMoO 4 . Wulfenite occurs commonly in yellow, orange, red, and grayish-white crystals. They may be tetragonal, tabular (see illus. ), or ...
pentlandite
Science; General science
A mineral having composition (Fe,Ni) 9 S 8 . Pentlandite is the major ore of nickel. It crystallizes in the isometric system, but crystals are rare. It is usually massive, showing ...
proustite
Science; General science
A mineral having composition Ag 3 AsS 3 and crystallizing in the hexagonal system. It occurs in prismatic crystals terminated by steep ditrigonal pyramids, but is more commonly ...
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