Black opal.
Opal which is found with a natural black or dark background. This background can range from pitch black to grey giving the stone are darkish appearance when seen from the top. It is the dark background which allows the brillant colours. Black opal can be any colour. The specimen pictured has a dominent colour of blue. It is approximately 1-1/2 cm in length.

Semi-Black opal.

White opal.

Crystal opal.
An opal with a brilliant crystal appearance allowing you to look down into the stone. The light background of this opal is translucent.

Jelly Crystal opal.
A solid crystal opal that is extremely translucent, to the point of being almost transparent. The sample specimen is from the Coober Pedy mines. It had to be photographed against a dark background to bring out its colour.

Boulder opal.
Similar to black opal except that the foreground colour is very thin and sits on a brown ironstone base. These stones at times can come with interesting "hills" and "valleys" on the surface.(meaning that the surface is often, but not always, undulating) These stones are preferred by people who are more "progressive" in their jewelery tastes.Often boulder opal has ironstone inclusions in the foreground and all sorts of odd shapes which makes them a designers delight.

Boulder opal matrix.
(matrix actually comes from the word "mother" or "womb", meaning that the ironstone is the "mother" that holds the "baby" which are tiny flecks of precious opal. A characteristic of boulder matrix opals is that the colour is usually peppered throughout the stone.

Andamooka Matrix.
A more porous opal found in the mining field of Andamooka. When it comes out of the ground it is quite pale but by treating it with a carbon dye process it eventually looks like real black opal. An Andamooka matrix opal can be an affordable alternative to a genuine black opal--but buyers beware--an honest opal dealer will tell you whether you are buying an Andamooka matrix or a genuine black; a disreputable dealer may not.

Yowah Nut.
Brillantly coloured opals nested in a nugget of ironstone. The specimen is a Yowah Nut with a black crystal opal. Though the opal center is sometimes separated from its ironstone nugget, many beautiful and unusual jewelry pieces are made by keeping the nugget and its center-opal together. (For a full history of this small unique Yowah opal mine, order Len Cram's book Beautiful Yowah.)

Opal fossils.
Opalized shells, crustations, sea creatures, snails, animal claws and bones, are some of the many objects that over time can opalize when buried in the unique environment of Australian clay and left undisturbed. These chunks of opal are marvelous conversation pieces for your mantlepiece or office desk and come with full explanation. Pictured is a cluster of opalized shells. These indicate evidence of an ancient flood.

Specimen.
A chunk of opal that is either left in its natural state or given a polish or varnish to show off one aspect or feature of the stone. Opal specimens are sought after by rock collectors, who generally display them on a desk or shelf.. The example is actually half of a specimen pair. With a specimen pair, both halves actually fit together, but are usually displayed side by side to show off the opal in the centre.

Mosaic Opal.
The previously described opals are all naturally formed. The mosaic opal is a man-made creation. Slivers of genuine opal are assembled into an irregular tiled pattern with rivers of black potch framing each unique opal tile. Mosaic opals make stunning stones for all types of jewelry and allow the jeweler to include a wide range of colours within an affordable piece. Now, there is a natural stone that looks very similar to mosaic opal. The natural phenomenon is called the harlequin opal. It is the rarest and most expensive opal.