Covellite
Covellite is copper sulfide, CuS, a hexagonal mineral instantly recognizable for its deep indigo-blue color and the flashing, rainbow-like iridescence that appears on its surface as it tarnishes. It forms as a secondary mineral in the supergene enrichment zones of copper sulfide deposits, where descending groundwater alters primary copper minerals. Despite its striking appearance, covellite is quite soft and delicate compared to most sulfide minerals collectors encounter.
Covellite properties at a glance
| Type | Minerals |
|---|---|
| Chemical formula | CuS |
| Mohs hardness | 1.5–2 |
| Crystal system | Hexagonal |
| Luster | Metallic to submetallic/resinous |
| Streak | Lead-gray to black |
| Cleavage | Perfect basal {0001} |
| Fracture | Uneven; sectile, flexible flakes |
| Specific gravity | 4.6–4.76 |
| Transparency | Opaque |
| Common colors | Indigo blue, Blue-black, Iridescent purple/red tarnish |
| Rarity | common |
Geology & background
Covellite typically develops in the oxidized, weathered upper portions of copper ore bodies, where copper-bearing solutions percolating downward react with existing sulfide minerals like chalcocite and bornite to deposit thin, foliated covellite coatings and crystal aggregates. Notable localities include Butte, Montana, once one of the richest copper districts in the United States, and Sardinia's Calabona mine in Italy, which produced particularly lustrous crystal specimens prized by collectors worldwide. Volcanic fumarole deposits, such as those at Vesuvius, can also produce covellite under very different high-temperature conditions.
Because covellite is so soft, specimens require careful padded storage to avoid scratches from harder minerals sharing the same box. Its indigo-blue color combined with a coppery, iridescent tarnish makes it a favorite among collectors of colorful sulfides, even though individual crystals are usually small and thin. Fine display pieces often show a play of purple, gold, and rose iridescent colors across cleavage surfaces, an effect caused by thin-film interference rather than any dye or treatment.
How to identify Covellite
Covellite is dark indigo-blue to nearly black, often overlaid with iridescent purple, gold, or bronze tarnish colors, and has a submetallic to metallic luster. Its streak is gray-black to lead-gray rather than white, a useful distinction from many similarly colored minerals. Hardness is low, around 1.5 to 2, easily marked even by a copper coin, and it shows perfect basal cleavage that produces thin, flexible flakes. It is often confused with bornite, but bornite's tarnish leans more purple-brown overall and its hardness is somewhat greater.
- Compare the specimen's color, luster, crystal habit, grain size, streak, and locality before confirming.
- Use multiple photos when possible, including a fresh surface and a close view of texture.
Where Covellite is found
Reported from multiple localities worldwide; verify locality against specimen provenance when available.
How much is Covellite worth?
| Entry-level specimen | $2 |
|---|---|
| Typical collector specimen | $20 |
| Premium specimen | $150+ |
Value depends on size, crystal quality, color, locality, rarity, damage, and provenance.
Common uses
- Collector specimens
- Education
- Field identification
- Industrial or lapidary use when suitable
Interesting facts
- Covellite's brilliant blue and iridescent surface colors come from thin-film optical interference, the same physical effect that colors soap bubbles.
- The mineral was first described from sulfur-rich fumaroles on Mount Vesuvius, an unusually high-temperature setting compared to where most covellite forms today.
- Despite looking metallic and tough, covellite is so soft that a copper penny can scratch it.
Care & safety
Handle as a collector specimen until hardness, cleavage, and fracture are verified.
Avoid inhaling dust. Do not cut, grind, or polish unknown specimens without eye protection and a proper respirator.
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