Black opals are the rarest of all the varieties of opal. In fact, all black opals mined today are sourced from only a few locations in the world- the most famous of which being Lightning Ridge, Australia.
Despite their name, the body of black opals is not always black. The term covers a range of dark coloured opals, including opaque and crystal.
The most sought after of these precious gems display a rich variety of colours and patterns, which is particularly vivid against the dark background of the stone.
(All images via http://www.opalauctions.com/)