Terminology [see www.opalmine.com-encyclopedia]
Pale Opal: All opals that are light in color without a dark background. Crystal, is high quality pale opal.
White Opal: Pale opal that is white-ish in color, that is not translucent.
Crystal Opal: Pale opal with brighter colors that are translucent. (I.e., you can see into the stone.)
Jelly Opal: Pale opal that is like jelly. (Very see-through) It can contain various degrees of color.
Jelly Crystal: A cross between Jelly and Crystal opal.
Black Opal: Opal with a pitch-black background, thus causing the foreground of the stone to have a darker appearance.
Semi Black Opal: Same family as black opal but with a grayer background. This can range from very light Grey, almost whitish background to quite dark. The darker the background directly beneath the color, the brighter and more defined is the foreground. -
Sunflash: Black opal that is very dark with just a hint of color coming through, usually red. Black Crystal: - Fig 18 (right) Black opal that is slightly translucent like crystal opal. The black or dark background is right through the stone instead of just in the background like black opal.
Rough Opal: Opal in its rough or uncut form.
Potch: Unformed Opal with no color play. This can be white, grey or black.
Seam Opal From Lightning Ridge that has been rubbed so as to expose the color.
Cutters can request this service from some dealers. This way there is a lot less risk
Seam Opal: Probably named because of the ‘flattish’ appearance of the rough. It is often found in horizontal ’seams’ or lines in the opal dirt. It is not always flat. It often comes in lumps like ‘Knobby’ opal. (See. Fig’s 19,20)
Seam Opal: Probably named because of the ‘flattish’ appearance of the rough. It is often found in horizontal ’seams’ or lines in the opal dirt. It is not always flat. It often comes in lumps like ‘Knobby’ opal. (See. Fig’s 19,20)
Colour Bar: A line or bar of colour going through the stone with potch on either side. This can be either straight or bent, horizontal or vertical. (Usually used in connection with South Australian ‘Seam’ opal.)
Knobby Opal: Similar to ‘Seam’ opal, only that the stone is more random in shape and in its position in the opal dirt. It is often lumpy in shape and can come in fossil crustaceans, called ‘Chinaman’s Hats’ (because of their shape).
Knobbies (although most are just black potch) are highly sought after by the opal miner, knowing that most famous black opals have been cut from them
.Fossil Opal: As the name implies, this type of opal can be opal replacements of bones, roots, animal claws or teeth. Many fossils are found in the Coocoran area of the Lightning Ridge field, but they are found in most fields.
Opal Shells: - These are the most common fossils found on the fields. Coober Pedy produced thousands of these out of the famous ‘Shell Patch’ field, but they are plentiful in the Lightning Ridge area as well. All these sea creatures in the desert are obvious signs of either a great flood or at least large inland oceans thousands of years ago.-
Coober Pedy is famous for it’s white opal and crystal opal shells. They are well sought after by collectors and the offcuts make very bright solid opals and doublets. Sometimes they are backed with boulder opal ironstone and look for the world like solid boulder opals, unless you know what to look for. Always be careful when buying a boulder opal to make sure that it is not really a Coober Pedy shell with a boulder back. A slight prick with a sharp needle on the side of the stone where the ironstone meets the opal, will indicate whether or not the back has been cemented on.
Boulder Opal: name suggests, this opal comes in fine seams embedded in the boulders which are called (probably incorrectly) ironstone. (More likely hardened desert sandstone.) These boulders can be as small as a pebble or up to a meter thick and are usually smashed into subjection with sledge hammers. (Fig. 13) A lot of opal is often destroyed in this process. Very large diamonds saws are sometimes used if the color is obvious. Cutting boulder opal is a very muddy experience.
Boulder opal fun stones (These are stones with ironstone showing in the opal face) can be sold from $10 each for stones with weak color and a lot of ironstone, to practically any price if the stone is exceptionally dark in the background and very bright with good patterns. However, it would be unusual to get more than around $10 000 for a stone like this unless it was very large and red.- Fig’s 35,36 (above)
Matrix Opal: (natural) Matrix, coming from the term ‘maternal’ or ‘mother’ indicates opal that comes as a part of other material that holds it as in a ‘womb’. There are two types of matrix opal thus: Boulder Matrix , which comprises matrix stones from all the Queensland fields such as Winton, Quilpie, Jundah, Yowah, and Karoit.
Andamooka Matrix: (treated) A natural porous stone containing mostly opal. The color is accentuated by a process of boiling it in sugar or glucose and sulfuric acid. The sugar soaks in to the porous rock and the acid turns the sugar into carbon, which of course turns the stone black, thus giving it a black opal appearance. Fig. 40 (top, right) a large specimen featuring a strip of orange- red standing out against the green.
Painted Lady: - Fig. 41 (left) A thin white-ish or crystal layer of opal color attached to a white or creamy colored rock found in Andamooka. It is possible to cut stones from it in a similar fashion to Queensland boulder opal but it is usually left as a specimen. In fact with a little imagination one could call it South Australia’s “boulder opal” without the stark colors familiar to boulder and the brown ironstone.
Pancakes: Found mainly in the Winton area. The name again is very descriptive. They are boulder opals, very flat-ish, often with very pretty bands of ironstone, and can achieve an excellent polish, similar to some Yowah nuts. The specimen left (Fig. 42) was cut from a Winton Pancake.
[see www.opalmine.com-encyclopedia]
Yowah Nuts: As the name implies, these are found in ‘nutlike’ formations, as small as a pea to approximately fist size. They are really miniature boulder opals but are famous for their fascinating bands of ironstone with associated natural matrix opal. The most sought after these has a kernel of pure opal, but also in big demand by collectors is the unusual patterns and pictures caused by the veins of opal in the ironstone.
[see www.opalmine.com-encyclopedia 'at the mines']
Where Opal is found in Australia see 
Faceting takes a lot more expensive equipment, is more difficult to learn, takes a lot of time, and in most cases the efforts are not rewarded because the job is already being done very cheaply by large well organized factories using cheap 
Note: There are quite a lot of differences between the Australian and American ways of spelling. I have opted in most cases to using the American, since most of my business is done in that country,. . and my word processor speaks American and keeps telling me that I’ve made a mistake…so I’ve decided to go along with it. Also keep in mind that for every suggestion made about gem cutting procedure, there will be someone who disagrees with it. If you think you’ve got a better suggestion, or would like to put your point of view, please take advantage of the chat program on the 
Unless you understand the various terms associated with the movement of light, you will not understand how various opals are identified and put into different categories or groups.
As an opal dealer, you will want to flatter your stones and present them in their best dress so as to speak. 
