The value of opal is defined by the brilliance of colours in the colour bar. The brighter and sharper the colours, the more valueable the opal is. If it is on flawless potch (see below), it is considered more valuable than on a mixed layer of potch.

There are several types of opal, with the name usually referring to the background of the stone. For instance, 'Black Opal' is in fact brilliant colours of opal on top of black potch.

	Potch - common opal with no colour. Varies from black to milk white to greenish, yellowish or bluish in colour. Potch has no real value in itself, its value comes from the effect it has on the colours above it. For example the same piece of coloured opal on top of black potch would have an entirely different look, and value, when placed on top of white potch.
	Black Opal - strong, brilliant colours on a flawless black background of potch. Any colour bar of opal on black or blue/black potch is considered true black opal. Australia has the finest black opal in the world.
	Black Crystal Opal - gem colours on a smoky, transparent background, some people regard this as the best type of opal ever found.
	Semi-Black Opal - gem colours on greyish potch backing, not in the same league as true black opal.
	Grey Opal - varies from light grey to dark grey and sometimes smoky in appearance. The colours can be very bright on this semi-dark backing of potch .
	Crystal Opal - strong dazzling colours in a more transparent stone with no potch background.
	Milky/White Opal - opal with all the usual colours on a milky white to greenish, yellowish or bluish background potch backing.
	Boulder Opal - ironstone, mudstone and sandstone boulders containing veins of opal from millimetres to centimetres in thickness.
	Matrix Opal - a porous sedimentary ironstone having minute cavities filled with precious opal. When cabochon cut, the dark brown ironstone sparkles with vivid pin flecks of colour.

In addition to the types of opal described above, each individual opal is then classified into one of three categories, basically depending on the thickness of the stone.

	Solid Opal - this is an opal as it is mined. It is of suitable thickness to be usuable as a single opal, or in jewellery. Depending on the type (as described above) it consists of coloured opal on top of a potch background - keeping in mind this is how nature formed it.
	Doublet Opal - sometimes we find brilliant colours in thin, or crystal opal. These opals are often turned into doublets, which are produced by glueing the opal to a natural black potch background, thereby simulating the effect of a true black opal. These doublets are truly investment opals, and are mostly found set in jewellery where it is impossible to tell the difference between real black opal which could be as much as 10 times the price of a doublet. Here at Opal Shed Mining Co we produce thousands and thousands of doublets each year using only the very best quality crystal opal and black potch. The glue we use is a special solution that is impervious to water, acids and solvents. So much so that our doublets carry a full lifetime guarantee against separation.
	Triplet Opal - is similar in concept to doublets, except these consist of a black backing, a thin layer of coloured opal, and then a clear quartz top. These are usually set in gold-plated fashion jewellery.