Bornite: Gemstone Information
Bornite is Cu5FeS4 and forms opaque compact copper red to bronze masses with a metallic lustre and pseudocubic crystals of the orthorhombic system with hardness 3–3.25 and SG 5.08. The fresh red to brown surface colour does not persist and an iridescent purple soon develops. The RI is out of range of gemmological testing. Bornite is occasionally fashioned into cabochons; the mineral occurs in mafic igneous rocks, in pegmatites and in contact metamorphic (skarn) deposits. Fine examples have come from the Redruth area of Cornwall, England, and from Butte,Silver Bow County, Montana, USA.