The best crystal opals are Australian crystal opals. A crystal opal is any kind of opal with a transparent, translucent, or semi-translucent body. This is called the “diaphaneity” (the quality of being transparent or translucent) of a stone. If you can see through an opal or light can pass through it, it is likely that it will be a crystal opal. Crystal opals can, and often do, display many colors of the spectrum.
The clarity of a crystal opal usually comes from its translucence. This often gives it a greater clarity and vibrancy of color than most other stones.
Where possible, crystal opals are cut in an oval shape. This tends to enhance the colors within. One should look for an oval crystal opal if possible. Other cuts may not show off the colors within to their best effect.
Boulder opals, although they may have a layer of translucence, are not considered crystal opals as they usually have an opaque ironstone backing.
The transparency and clarity can affect the price by the enhanced appearance, giving the crystal opal a higher value than, say, an opaque opal. You can pay up around 1800 USD per carat for top quality crystal opals. Some can be 3 or 4 carats so the best crystal opals are up there with the diamond price also.
Crystal opals are mined in the states South Australia and New South Wales, in Australia. This are the same places black and white opals are found. The majority are found in the Coober Pedy area.
You can buy Australian crystal opals online of course. You would need to ensure that the dealer is selling genuine crystal opals, especially with the prices one can pay. Locating a dealer who is established or located in Australia and preferably in the Cooper Pedy area is a good start.
Do a comparison between dealers. Although no two crystal opals are the same it is still possible to do a comparison on the basis of size, weight, color and price. Also as pictures you see on the net are not always accurate, check to ensure that the dealer is willing to accept a return and refund your money if you are not satisfied.
Doing the above you means you can get a really nice crystal opal from source and such Australian crystal opals are very beautiful and can look wonderful in jewelery such as pendant or earrings.