Cobaltite is an important cobalt ore mineral having the formula (Co,Fe)AsS - cobalt iron arsenic sulfide. It's essentially arsenopyrite with cobalt. Cobaltite has a metallic luster, a bright silvery color, a dark gray streak, and is moderately hard (H = 5.5). It can form cubic and pyritohedral crystals (the same crystal forms as pyrite), but cobaltite has cleavage, unlike pyrite. It frequently occurs as finely granular masses mixed with other minerals (see below). Cobaltite occurs in some hydrothermal veins, and in some contact metamorphic rocks.
Cobaltite (silvery-colored) mixed with calcite (whitish-grayish) (4.3 cm across at its widest).