Covellite Identification Guide

What Covellite Looks Like

Covellite (copper sulfide, CuS) is an indigo-blue to dark blue or violet metallic mineral, often with a striking iridescent purple, bronze, red, or gold tarnish that flashes when tilted. Luster is metallic to submetallic, dulling to resinous on cleavage faces. It is opaque. Crystals (hexagonal) are usually thin platy, tabular, or foliated, but covellite more commonly occurs as massive coatings, films, and disseminations in copper ore. A diagnostic feature is its perfect basal cleavage producing thin, flexible (slightly elastic) blue flakes, like a soft blue mica. Wetting a surface can intensify the purple color.

Step-by-Step Field ID Checklist

1. Spot the blue. Indigo to dark blue body color is the headline trait.
2. Look for iridescence. Purple, bronze, and gold play of color when tilted.
3. Test cleavage. It splits into thin, flexible blue flakes (perfect basal cleavage).
4. Check softness (below).
5. Take a streak. Dark gray to black streak (see tests).
6. Note the setting. Associated with other copper sulfides (chalcocite, bornite, pyrite) in copper deposits.

Key Diagnostic Tests

• Hardness: 1.5–2, very soft; easily scratched by a fingernail-to-knife.
• Streak: Dark gray to black (sometimes shining), not blue — a key check.
• Cleavage: Perfect basal {0001}, yielding thin flexible (not brittle) plates.
• Magnetism: None.
• Acid: Soluble in nitric acid (lab test); not needed in field.
• Density: Moderately high, ~4.6–4.8 g/cm³.
• Wet test: Color often deepens to vivid purple when wetted.

Common Look-Alikes and How to Tell Them Apart

• Bornite ("peacock ore"): Iridescent but on a brassy-bronze base, brittle, harder (3); covellite is blue-bodied, very soft, and has flexible micaceous cleavage.
• Chalcocite: Dark gray to sooty black, lacks covellite's blue color and flexible flakes.
• Azurite: Bright blue but a carbonate that fizzes in acid, is harder, and has glassy (not metallic) luster.
• Molybdenite: Soft and flexible too, but bluish-silver-gray (not indigo) with a greenish streak.
• Iridescent hematite/limonite films: Earthy, harder substrate, no flexible blue cleavage flakes.

Where It Is Typically Found

Covellite forms in the supergene (secondary) enrichment zone of copper sulfide deposits, as a late alteration of other copper minerals. Notable localities include Butte, Montana (fine crystals), Leogang in Austria, Sardinia (Calabona, the type area), and copper districts worldwide; it typically coats or replaces chalcocite, bornite, and chalcopyrite.