The rarest and most valuable of all opal comes from the Lightning Ridge area and is known as Black Opal. It can also be found in certain fields in Coober Pedy and Mintabie South Australia.
It gets its name from the darkness of the opal body. A common misconception is that the whole stone is completely black, but this is not accurate. The term simply means the stone has a dark body tone in comparison to white opal. It is this darkness which accentuates the play of colours within the opal, which is where the value lies. If the stone was completely black it would simply be classified as black potch opal, relatively worthless compared to its colour bearing counterpart.
The colour tone of Australian black opal can range from dark grey to jet black, see body tone chart. The dark colour of the opal is caused by the presence of carbon and iron oxide trace elements.
Tone chart (opals are placed on the chart to match their tone)
The darker the body tone of the opal the greater the vibrancy of colour within the stone due to the natural highlighting nature of colour against black. Colour flashes stand out more in a black opal than in white or light bodied stone which is why black opal is more highly valued by collectors. It is on this principle that doublets and triplets are made. A crystal opal is placed on a black potch backing which then accentuates the play of colour within the crystal opal that otherwise may be difficult to see.
Lightning Ridge black opal
It’s interesting to note that even expensive black opal may have only a very thin colour bar on black potch.
Black opal can be found as seam opal which is formed in the fissures and cracks of the ground but more commonly is found as “nobbies”. Nobbies are isolated pockets of rounded opal that may have formed due to a clay cavity within the earth being filled with silica laced water in turn forming into opal. Some nobbies are believed to be opalised fossils of sponges and coral.
Black opals are often cut into an oval shape, however this depends on the natural formation of the colour bar within the stone. Free form and tear drop shapes are sometimes used to maintain maximum size and carat weight of the stone. Black opals are normally cut with a low cabochon due to the opal colour bar being much thinner in black opal than white or crystal opals.
Top of the range, gem quality Australian black opal can be valued in excess of AUD$15,000 per carat. This makes Australian black opal the most expensive of all opal. However, just because an opal is black doesn’t make it valuable. Multiple factors influence opal valuation including but not limited to, brightness, pattern, imperfections and finished shape. For more information on opal valuation please see our blog post.
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