Smithsonite

Smithsonite is zinc carbonate, a member of the calcite group and an attractive secondary ore of zinc. It rarely forms sharp crystals; instead it is best known as botryoidal, reniform, and stalactitic crusts with a smooth, glassy to silky, almost porcelain-like surface.

Trace elements give smithsonite a wide palette — copper produces its prized blue-green and apple-green, cobalt gives pink, cadmium and iron yield yellow (the "turkey fat" variety), and pure material is white. The mineral was named for James Smithson, the founder of the Smithsonian Institution.

Historically smithsonite was confused with hemimorphite under the old name calamine, and both were important early sources of zinc.

Smithsonite is a secondary mineral that forms in the oxidized weathering zone of zinc ore deposits, where carbonated surface waters alter primary sphalerite (zinc sulfide). It precipitates as crusts and replacement masses, especially where the ore sits in or near limestone and dolomite host rocks that supply carbonate.

It commonly occurs with hemimorphite, cerussite, malachite, azurite, and limonite in oxidized ore bodies. Outstanding localities include Tsumeb in Namibia, Kelly Mine and the Magdalena district in New Mexico, Choix in Mexico, Lavrion in Greece, Sardinia in Italy, and Broken Hill in Zambia, which produced gemmy blue-green material.

Smithsonite is recognized by its botryoidal, grape-like or stalactitic crusts with a smooth, glassy to silky luster and characteristic pastel colors — especially blue-green, apple-green, pink, and yellow. It has a white streak, hardness 4-4.5, and is distinctly heavy for its appearance because of its zinc content.

It effervesces in warm dilute acid like other carbonates. Its high density is a key clue separating it from lighter look-alikes such as chrysocolla, hemimorphite, and prehnite. Hemimorphite (the other "calamine") is lighter and often forms bladed crystals; smithsonite's weight, botryoidal habit, and acid fizz help confirm it.

Smithsonite has long been an important ore of zinc, mined under the old name calamine before zinc smelting was fully understood; zinc from such ores went into brass and galvanizing. Where it occurs in quantity it remains an economic zinc source.

Gemmy, richly colored botryoidal smithsonite — especially the blue-green Kelly Mine and Tsumeb material — is highly prized by mineral collectors and occasionally cut into cabochons, though its softness limits jewelry use. The vivid blue-green variety is sometimes sold as "bonamite." Metaphysically it is associated with calm and emotional balance, but its primary significance is as zinc ore and collector mineral.