Dioptase
The green that persuaded eighteenth-century Moscow it had found emerald. Hardness 5, perfect cleavage, and a poor traveller.
Dioptase is hydrous copper silicate, CuSiO3·H2O, hardness 5, forming emerald-green rhombohedral crystals in the oxidised zone of copper deposits. Haüy named it in 1797 from the Greek for seeing through, after the cleavage planes visible inside the crystals. Tsumeb, the Kaokoveld and Altyn-Tyube are the classic localities.
In short
- CuSiO3·H2O, hardness 5, trigonal, with perfect rhombohedral cleavage — which is why chipped crystals are so common in the trade.
- The colour is copper in the structure itself, not a trace impurity, so there is no pale variety and no documented fading.
- Tsumeb in Namibia gave the largest lustrous crystals; the Kaokoveld gives bright green crystals in white dolomite; Altyn-Tyube in Kazakhstan is the type locality.
- It is not emerald. Beryl is hardness 7.5–8 with no easy cleavage; dioptase is far softer and splits on three directions.
- Rarely cut. A faceted dioptase is a collector's curiosity, not a wearable stone, and most crystals are under 2 cm.
| Species | Formula | Typical appearance | Hardness |
|---|---|---|---|
| Dioptase | CuSiO3·H2O | Emerald to blue-green rhombohedral crystals | 5 |
| Malachite | Cu2(CO3)(OH)2 | Banded, botryoidal or fibrous green | 3.5–4 |
| Chrysocolla | Hydrous copper silicate, variable | Blue-green, massive, often porous | 2–4 |
| Brochantite | Cu4(SO4)(OH)6 | Emerald-green needles and sprays | 3.5–4 |
| Atacamite | Cu2Cl(OH)3 | Dark green slender prisms | 3–3.5 |
| Conichalcite | CaCu(AsO4)(OH) | Yellow-green botryoidal crusts | 4.5 |
| Emerald (beryl) | Be3Al2Si6O18 | Green hexagonal prisms | 7.5–8 |
The emerald that was not: Haüy, 1797
Dioptase reached Europe from the Kirghiz steppe in the late eighteenth century, traded out of Central Asia and sold in Moscow as emerald. The mistake was reasonable — nothing else in circulation was that green in transparent crystals — and it survived until the material was tested properly. It scratched at hardness 5 where beryl scratches at 7.5 to 8, and it split cleanly on three directions where emerald does not.
René Just Haüy named it in 1797, from Greek roots meaning to see through, because those internal cleavage planes are visible inside an intact crystal when it is held to the light. The name records the diagnostic feature and the disappointment at the same time.
The source was Altyn-Tyube — "golden hill" — in what is now the Karaganda region of Kazakhstan, an oxidised copper occurrence worked for copper in prehistory. It remains the type locality, and Altyn-Tyube crystals in old-collection condition still turn up, usually small, dark and on a pale carbonate matrix.
Tsumeb and the Kaokoveld
Tsumeb produced the dioptase that defines the top of the species: dark, saturated, highly lustrous crystals, the best of them well over a centimetre and exceptionally transparent for the mineral. Tsumeb closed its main workings in 1996, so every fine Tsumeb dioptase now on the market is out of an old collection, and a documented label is a material part of what you are buying.
The Kaokoveld in northwestern Namibia is the other Namibian source and the one most collectors actually own. Crystals there sit in and on white to cream dolomite, and the contrast between bright green and pale carbonate makes for the most immediately attractive dioptase specimens available at ordinary sizes. They are typically smaller and brighter than Tsumeb material, and more often in clusters and druses than as isolated crystals.
Namibian dioptase associates with calcite, dolomite, duftite, cerussite and wulfenite depending on the occurrence, and a plate with a second species on it is generally worth more than the same plate without.
Altyn-Tyube, the Congo and Arizona
Beyond Namibia, the Republic of the Congo has been a steady source — Mindouli and Rénevillé in particular — producing crystals on and in quartz and chrysocolla, generally smaller than Namibian material but sometimes in very rich coverage. Tantara in the Katanga copper belt has also produced dioptase.
In the United States, the Christmas mine in Gila County, Arizona produced dioptase in small, sharp, dark crystals that are prized well out of proportion to their size because American dioptase is scarce. Chile, Peru and Iran have each given occasional material.
The pattern is consistent and worth understanding before you buy: dioptase forms where an oxidising copper deposit sits in or near carbonate rock in an arid climate, because it needs both copper and silica in solution and needs the oxidised zone preserved rather than eroded. That is why the map is so short.
Cleavage: why dioptase arrives damaged
Dioptase has perfect rhombohedral cleavage, and at hardness 5 it is stiff enough to feel robust in the hand while being easy to split with a knock. This is the single most important thing to know about the species. More dioptase is ruined in transit and in drawers than was ever ruined underground.
Inspect the terminations with a lens, not the body of the crystal. A cleavage break shows as a flat, bright, mirror-like face at an angle unrelated to the crystal's natural faces, and it catches the light differently from a growth face. On a druse, run your eye across the whole plate at a raking angle; fresh breaks announce themselves as a scatter of bright points.
Pack it in its own compartment, in soft tissue rather than loose foam chips, and never let a second specimen share the box. Repairs are common on important pieces and, if disclosed, are not a reason to walk away.
What a good dioptase looks like
Four things move a dioptase specimen. Transparency first: the mineral runs from near-opaque dark green to genuinely gemmy, and a crystal you can see into is a different object from one you cannot. Then lustre, which should be vitreous and bright rather than dull. Then colour, where the ideal is a deep saturated green that is neither blackened nor washed toward blue. Then, last, size — and only last, because a sharp gemmy 8 mm crystal beats a dull 2 cm one every time.
Matrix does a lot of work at the display end. Green on white dolomite is the classic Kaokoveld presentation and is worth paying for. A loose crystal, however fine, is a study piece.
Cut dioptase exists and is bought by gem collectors who want the species represented. It is not jewellery: hardness 5 and perfect cleavage mean a ring stone would not survive ordinary wear.