Types of opal
This is certainly the rarest and most valuable of all opal. The name ‘black’ refers to the base colour of the opal which is jet black to very dark grey. In technical terms N1 to N4 on the N-rating chart. See "How opal is valued"
Semi black or dark opal:
This is a sub category to black opal, as the name suggests, this is a lighter version of a black opal.
This refers to opal with a light background; the opal is either opaque or translucent. It is not quite as rare as black opal but still has a lively play-of-colour with bright to pastel colours schemes.
Crystal Opal comes in a vast variety of colours. They are either transparent or translucent and occasionally they can be polished on both sides. The body of a crystal opal is transparent enough to see through the body, when held against a dark background, the true beauty is revealed as the colour springs to life, showing their true potential.
Black Crystal Opal:
Black crystal has the same characteristics as a crystal opal, however, a black crystal show traces of black opal through out the body of the opal. It shows a dark, but, transparent body which causes the colours to play vividly.
Boulder opal consists of a layer of gem opal that has been natural deposited onto ironstone rock. Usually, the thinner the gem layer, the brighter the play-of-colour. It is said that the colour is more concentrated and therefore more intense.
Jelly or water opal:
This is opal that is colorless and transparent with little or no play of color, it may show opalescence without a pattern.
This is a man made opal consisting of a very thin layer of gem opal bonded with glue to a backing of black glass, black potch (non precious opal) or ironstone. The black background enhances the play-of-colour thus simulating quality black opal. Doublets have no investment value and usually cost much less that solid precious opal. Doublets can be damaged beyond repair if submerged in water for a period of time. See “A guide to opal care”
This type of opal is basically the same as a doublet with the addition of a domed cap added over the gem opal to enhance the play-of-colour by magnifying the colours. Again, a triplet is normally inexpensive as only a very thin slice of opal is used. It should not be submerged in water for to long as it could be damaged easily. See “A guide to opal care”
Mosaic opal utilizes the smaller coloured chips of opal which would otherwise be discarded. Instead, the irregular pieces of opal are tightly packed together to create a mosaic of coloured opal.
Synthetic and Simulated Opal:
These opals are opals that have been created in a laboratory setting. "Gilson" created opal was the world’s finest laboratory grown opal.