Cuprite

“Deep red crystals of copper oxide”

Mineral

Cuprite is a copper oxide mineral with a deep crimson to ruby-red color and an exceptionally high refractive index that gives gem-quality crystals a brilliant adamantine luster. It forms in the oxidation zones of copper deposits and was once an important copper ore in its own right.

Composition

copper(I) oxide (Cu₂O)

Where Found

Onganja (Namibia), Chessy (France), Bisbee (Arizona, USA), Cornwall (England), Rubtsovsk (Russia)

Mohs Hardness

3.5-4

Fun Fact

Gem-quality cuprite has a refractive index higher than diamond, giving properly cut stones an extraordinary fiery brilliance — but at hardness 3.5-4, they're too soft for any practical jewelry.