Rock of the Month

Malachite
Cu₂CO₃(OH)₂

While color may be misleading when it comes to identifying some minerals, with malachite, it’s a sure bet! All malachite is green, often a bright emerald green. It also leaves a green streak on a ceramic streak plate.

Malachite, a hydrous copper carbonate, is part of the large family of carbonate minerals. It’s very close, chemically, to azurite, Cu₃(CO₃)₂(OH)₂, which forms when copper weathers. When cool, oxygen-rich groundwater seeps into rocks (particularly limestone) that contain copper, it creates an “oxidation zone” that alters the copper, changing it to azurite. If weathering continues, azurite turns into malachite. Therefore, both these minerals are referred to as “secondary ores of copper” and can be mined for their copper content. Malachite most often occurs as crusts or botryoidal (bubbly-looking) masses with swirling bands of different shades of green. It sometimes forms as velvety fibrous masses, stalactites and, rarely, small crystals.

Famous sources of malachite include copper-mining districts in Zaire and Namibia and the Ural Mountains of Russia. It’s also found in copper-producing regions of the American Southwest and Mexico.

While it is relatively soft (Mohs 3.5-4) and often has an earthy or silky luster, malachite can take a fine polish and makes beautiful banded green cabochons, beads, inlay and sculptures. I have a malachite dolphin on my desk, and there’s a “Malachite Room” with huge green pillars in the Hermitage Museum in Russia. However, malachite dust can be toxic if it’s breathed in. When grinding and polishing malachite, keep the rock wet to reduce dust and wear a face mask.

—Jim Brace-Thompson

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