Black opal are the most valuable and they have a dark black, grey, blue, green or brown background that makes the play-of-colour flashes stand out the most. Unlike ordinary opals, black opals have carbon and iron oxide trace elements present, which cause the unusual darkness of the stone. Can be fragile which makes it more suitable for jewellery such as necklaces, brooches and earrings that are less likely to be knocked. Opal is the birthstone for October. Mohs Scale of Hardness: 5.5 - 6.5 Sources: Australia, Ethiopia and Brazil. The finest quality and most durable opals generally come from Australia.;Stone Size: 17 x 13;Carat Weight (ct): 10.69