Crocoite

Crocoite is a mineral made of lead and chromium (lead chromate, PbCrO₄). It forms bright red-orange, long, slender, needle-like crystals. It is an uncommon mineral that forms in the parts of lead deposits exposed to air, where the deposits lie next to chromium-bearing rocks. It was found in Russia's Ural Mountains in the 1760s, and the element chromium was discovered in its specimens in 1797. Its name comes from krokos, Ancient Greek for “saffron”. The finest specimens in the world come from the Australian island of Tasmania.

Thanks to crocoite, the orange-red mineral found in the Ural Mountains in the 1760s and known as “Siberian red lead”. The French chemist Louis Nicolas Vauquelin treated crocoite specimens to obtain chromium oxide first, and in 1797 he isolated metallic chromium. He is therefore regarded as the discoverer of chromium.

The discovery of chromium: The element chromium was discovered in crocoite specimens. Pigment: After Pallas recognised its potential in the 18th century, crocoite was used as a paint pigment in the Ural region. Today's chrome yellow pigment, however, is made artificially from the more plentiful chromite. Collecting: Its bright red-orange sprays of crystals make it stand out among collector minerals.

It has a hardness of 2.5–3 on the Mohs scale. A fingernail cannot scratch it, but a steel knife can. Its specific gravity is 6–6.1 g/cm³.

Its best-known localities: Berezovsk mines, near Yekaterinburg, Ural Mountains, Russia; Dundas, Tasmania, Australia; Leadhills, Lanarkshire, Scotland.

Forms bright red-orange or orange, long prismatic or needle-like crystals, often in radiating sprays or tangled clusters. Its streak (the powder it leaves on unglazed porcelain) is yellow-orange. It has an adamantine lustre; the crystals are transparent to translucent. Orange-red vanadinite and wulfenite also occur in lead deposits; vanadinite, however, forms hexagonal prisms and wulfenite thin square plates, while crocoite forms long, slender, needle-like crystals.

On the “street to diamond” value ladder it sits on step 4 of nine: Fine dimension stone and ore. Crocoite is valued not as an ore but as a collector's mineral. Collectors look for vividly coloured, lustrous, slender and unbroken sprays of crystals, especially from Tasmania's Adelaide and Dundas Extended mines. Because the crystals are very delicate and can lose their brilliance under ultraviolet light, specimens are stored with care, away from light.

Crocoite is toxic: it contains both lead and hexavalent chromium. Lead builds up in the body and can permanently harm the brain and nervous system, especially in children; according to the World Health Organization, there is no level of lead exposure known to be without harmful effects. Wash your hands after handling specimens and keep them out of reach of children. Do not put crocoite in your mouth, grind or heat it, put it in drinking water or breathe its dust.