Cuprite

Cuprite is a mineral composed of copper(I) oxide Cu₂O, and is an important ore of copper.

Its dark crystals with red internal reflections are in the isometric system, appearing as cubic, octahedral, or dodecahedral forms, or in combinations, with a deep red color. In spite of its nice color it is rarely used for jewelry because of its low hardness of 3.5 -4.