Australias National Gemstone, the most valuable and popular of all Opals: Black Opal. Black Opal is incredibly rare and is found at Lighting Ridge in Northern NSW. It is called Black Opal because it has a black base caused by black or grey iron oxide in the Opal base tone. This black potch or common Opal has no commercial value however 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 Black Opal comes in all the colours of the rainbow with red being the rarest and most expensive and may be designated N1, N2, N3 or N4 on the Scale of Body Tone.