oxides
Cuprite
Cu2O
- Also known as
- ruby copperchalcotrichite
- Crystal system
- cubic
- Hardness
- 3.5–4 on the Mohs scale
- Lustre
- adamantine
- Colour
- Deep red, sometimes almost black
- Where it forms
- In the oxidised zone of copper deposits, with malachite and azurite.
- Chiefly from
- Onganja, NamibiaChessy, FranceArizona, United StatesRussia
Why it is that colour
Copper oxide. Transparent crystals are a remarkable deep red and are among the most beautiful minerals there are.
Safety
A copper oxide, soft and reasonably reactive. Safe to handle; do not powder or put in water.
Where it comes from
A copper ore of secondary importance. Namibian specimens from Onganja, found in the 1970s, are the finest known and the deposit was small and quickly exhausted.
Historical use
A copper ore. Cuprous oxide was used in the first solid-state rectifiers, before silicon, and cuprite is therefore part of the prehistory of semiconductors.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
Worth knowing
Cuprite has a genuine claim on the history of electronics. Copper oxide rectifiers, made from this compound, were among the first practical solid-state devices — used from the 1920s in power supplies and instruments, decades before the transistor. They worked and nobody fully understood why, which was part of what drove the semiconductor physics that followed.
It is also a very good example of a mineral whose finest specimens came from a deposit that lasted a few years. Onganja cuprite is famous, expensive and effectively finite.
Identity checked against IMA-CNMNC master list, June 2025.