Dark Opal is Natural Gem Opal from Lightning Ridge in N.S.W., Mintabie and in some instances at Coober Pedy Australia. It is called Australian Dark Opal because it has a base tone in the caused by black or grey iron oxide in the Opal base tone which registers as N5-N6. This is a part of the colour bar that is often cut to be displayed on the surface of the Opal. The color bar or the play of colour of Australian Black Opal comes in all the colours of the rainbow with red being the rarest and most expensive. Dark Opals with Gem colour and a recognisable pattern are arguably the number 1 rarest commecially available gemstones found worldwide and are arguably 5,000x rarer than Diamond.