Crystals take the form of cubes but are uncommon. Usually found as grains, nuggets, and scales, platinum is silvery gray to white in color. The streak is white to silvery gray. Platinum is opaque and has a metallic luster. This luster is not altered by tarnishing if the mineral is exposed to the atmosphere.

FORMATION Originally formed in mafic and ultramafic igneous rocks, and rarely in contact aureoles, platinum also occurs in placer sediments because of its very high specific gravity.

TESTS When there are iron impurities present, platinum can be weakly magnetic. It is insoluble in all acids except aqua regia.

Group: NATIVE ELEMENTS

Composition: Pt

Hardness: 4–4½

SG: 21.44

Cleavage: None

Fracture: Hackly