Cuprite twinned crystals. The mineral cuprite is a pure copper oxide (Cu2O), formed in the oxidation zone of copper deposits. Its crystal system is cubic hexoctahedral. It forms well-developed crystals that are cubic, octahedral (see C017/9266) or dodecahedral. The paired, interlocking crystals in this sample are an example of penetration twinning. Colours may be black, red or green. This specimen is 11 centimetres across.