Cuprite

Cuprite is an oxide mineral composed of copper(I) oxide Cu2O, and is a minor ore of copper. Cuprite was first described in 1845 and the name derives from the Latin cuprum for its copper content.

Its dark crystals with red internal reflections are in the isometric system hexoctahedral class, appearing as cubic, octahedral, or dodecahedral forms, or in combinations. Penetration twins frequently occur. In spite of its nice color it is rarely used for jewelry because of its low Mohs hardness of 3.5 to 4. It has a relatively high specific gravity of 6.1, imperfect cleavage and a brittle to conchoidal fracture. The luster is sub-metallic to brilliant adamantine. The "chalcotrichite" variety typically shows greatly elongated capillary or needle like crystals forms.

It is a secondary mineral which forms in the oxidized zone of copper sulfide deposits. It frequently occurs in association with native copper, azurite, chrysocolla, malachite, tenorite and a variety of iron oxide minerals. It is known as ruby copper due to its distinctive red color.

Source: en.wikipedia.org/wiki/Cuprite

Cuprite Properties

Chemical Composition: 
Oxide of Copper - Cu2O
Colors / Varieties: 
Dark purplish red to brownish red to nearly black with a metallic appearance.
Crystal System / Forms: 
Cubic System
Hardness: 
4
Specific Gravity: 
5.85 - 6.15
Cleavage / Fracture: 
Poor, indistinct / Uneven fracture
Optic Character: 
Isotropic
Lustre: 
Adamantine to sub-metallic
Refractive Index / Birefringence: 
2.85
Pleochroism: 
Nil
Magnification: 
Natural inclusions.
U.V. Fluorescence: 
Inert
Spectrum: 
Not characteristic
Cause of Color: 
Copper
Specific Tests: 
Brownish red streak, soluble in hydrochloric and nitric acid.
Simulants (with separation tests): 
Heated Almandine Garnet (S.G., spectrum)
Geological Occurrence: 
Secondary mineral in copper deposits, sometimes coated with green malachite.
Sources: 
Namibia, Russia, Japan, U.S.A. (Arizona, New Mexico).
Cuts & Uses: 
Mostly as cabochons and beads.