Cobaltite

Cobaltite is a silver-white mineral with a metallic lustre, made of cobalt, arsenic and sulfur (CoAsS). Its crystals often look like cubes or pyrite crystals. About 45% of its weight is arsenic, so it is poisonous. Although it is one of the minerals mined for cobalt, most cobalt today is a by-product of copper and nickel mining.

Cobalt metal: According to the USGS, about 51% of the cobalt used in the United States goes into superalloys (mainly aircraft gas turbine engines), 25% into chemical uses and 9% into cemented carbides for cutting and wear-resistant uses. According to Wikipedia, the largest demand today comes from lithium-ion batteries. Blue colour: Cobalt compounds give glass, ceramics and paint a vivid blue. History: Indian jewellers used cobaltite to make blue enamel on gold and silver jewellery.

It has a hardness of 5.5 on the Mohs scale. A fingernail cannot scratch it, but a steel knife can. Its specific gravity is 6.33 g/cm³.

Its best-known localities: Tunaberg, Södermanland, Sweden; Skutterud (Blaafarveværket), Modum, Norway; Botallack mine, St Just, Cornwall, England; Cobalt, Ontario, Canada; Khetri, Rajasthan, India.

It is silver-white, often with a reddish tint, and has a metallic lustre. The streak (powder colour) is greyish black. Its hardness is 5.5 (a steel knife scratches it only with difficulty) and its specific gravity is about 6.3, so it feels heavy. It splits perfectly in one direction and is brittle. Crystals are often cubes or pyrite-like forms (pseudo-pyritohedra), and their faces can be striated as on pyrite. Compact or granular masses also occur. Cobaltite has no widely known imitation. It can be confused with similar silvery-grey, heavy, metallic arsenic minerals (such as arsenopyrite or skutterudite); because they are hard to tell apart by eye, analysis is needed for a sure identification.

On the “street to diamond” value ladder it sits on step 4 of nine: Fine dimension stone and ore. Cobaltite is an ore, and its value depends on the cobalt grade of the ore, the cost of separating harmful components such as arsenic, the size of the deposit and demand for cobalt (above all from the battery and superalloy industries). According to the USGS, the great majority of the world’s cobalt resources lie in sediment-hosted copper deposits in Congo and Zambia, in nickel laterites and in magmatic nickel-copper sulfide deposits; cobaltite is not singled out as a leading ore mineral among these deposit types. For collectors, well-formed cubic or pyritohedral crystals, freedom from damage and famous localities such as Tunaberg or Cobalt (Ontario) count. Because it contains arsenic, it must be stored safely.

Cobaltite is about 45% arsenic by weight; it is poisonous if swallowed or if its dust is breathed. Do not lick it, put it in your mouth, or put it in water to make “stone water” or an “elixir”. Do not heat it (arsenic-bearing fumes are released), crush, grind or sand it. Specimens with a pink bloom of erythrite on the surface also contain arsenic. Wash your hands after handling and keep specimens in a closed container, away from children and pets.