steel 5.5 gray black rose color in smaltite nitric acid 4.5 steel gray very heavy platinum 4 reddish black often in striated manganite prisms 1 gray with greasy feel graphite silvery 5.5 black arsenopyrite 2.5 silvery tarnishes black on silver exposure reddish 5.5 gray black rose color in cobaltite nitric acid pearly 1-1½ shining exposed surfaces cerargyrite violet brown White, with 4 white porcelainous magnesite impurities masses, effervesces in acid grayish 2 white earthy masses, kaolinite or greasy feel yellowish 1-3 white earthy masses bauxite 1 white greasy feel, talc fibrous or scaly TRANSPARENT OR TRANSLUCENT COLORS Color Hardness Remarks Mineral Colorless or with faint tinges of color due to impurities 10 in octahedrons diamond 9 in hexagonal prisms corundum 8 in hexagonal prisms topaz 7 in three-sided prisms tourmaline 7 in hexagonal prisms quartz 7 non-crystalline chalcedony 7 or 4.5 cubes with beveled edges boracite 6 non-crystalline, pearly luster opal 5.5 rhombohedrons willemite 5.5 trapezohedrons analcite 5.5 tufts of needle-like crystals natrolite 5.5 sheaf-like bundles of crystals stilbite 5 hexagonal prisms with basal cleavage apatite 5 effervesces in acid smithsonite 5 becomes jelly-like in acid calamine 4.5 monoclinic prisms colemanite 4 in cubes fluorite 3.5 effervesces in acid, but one cleavage aragonite 3.5 effervesces in acid, heavy cerrusite 3 effervesces in acid, rhomboidal calcite cleavage 3 no effervescence, but soluble in anglesite