Carrollite

Carrollite is a rare copper‑cobalt sulfide mineral, with chemical composition of copper cobalt sulfide (copper, cobalt, nickel sulfide), ideally CuCo₂S₄, although substantial nickel and iron substitutions frequently occur in natural specimens It crystallizes in the isometric (cubic) system, commonly forming octahedral or cubic crystals, as well as massive granular aggregates. The mineral displays a metallic luster and opaque appearance, often light to dark steel-gray, occasionally tarnishing to copper-red or violet-gray. Strength and brittleness characterize its physical behavior, with cleavage that is imperfect or indistinct and fracture that may be conchoidal or uneven.

Geologically, Carrollite forms in hydrothermal vein deposits and stratiform copper–cobalt–nickel sulfide environments, often associated with minerals such as chalcopyrite, bornite, digenite, djurleite, linnaeite, siegenite, polydymite, tetrahedrite, pyrite, sphalerite, millerite, gersdorffite, ullmannite, and cobalt-bearing calcite It was first discovered in 1852 by William L. Faber but in 2001, the Katanga Kambove District gained notoriety for producing the world’s finest Carrollite specimens with exceptional luster. These crystals are commonly recovered in association with calcite (calcium carbonate) and chalcopyrite (copper iron sulfide), often perched on calcite matrices in display specimens.