Natural black Opals 10x8mm Oval Flat Backs Gemstones From African Mines Natural Opals Oval Flat Backs Gemstones From African mines in all sizes from 6x4mm to 10x8mm perfectly calibrated. These are perfect to make pendants and jewelry! Each piece is handpicked with a wonderful play of colors Opal is one of the official birthstones for the month of October (along with pink tourmaline) and an alternative birthstone for April. It is modernly used to mark the 14th anniversary of marriage. Mineral Information : Chemical CompositionSiO2-nH2O ColorColorless, white, yellow, red, orange, green, brown, black, blue to waxy Hardness5-1/2 to 6 (Mohs) Specific Gravity1.98 - 2.20 Refractive Index1.450