Opals have different refraction from different angles. The more refraction from different angles, the more colors, and the more refraction coverage over the stone, the more valuable the opal. Look for sellers who have more than one picture from different angles.
There are three basic categories of opals that are on the market. 1. Solid opals, which mean that they are cut from a single stone, and they may have bits of the potch, the stone the opal was in. It does not detract from the overall value of the stone if there is some potch still in or on the opal. The solids also include boulder opals, which are small pockets or lines of opal within the mother stone, and are considered solid opals, since they are mined in this form. Crystal opals are also solid opals, they are transparent, with refraction within the clear stone. They are a wonderful item, and quite rare with colors other than blue. The darker an opal is, the more valuable, and then within each shade of opal, the more colors, the more valuable, and within the colors, the more flash, the more valuable. 2. A doublet. This means there is a sliver of opal, glued to some dark stone base, kind of like a mirror is made. The understone then makes the opal very much flashier than it actually is. These are a lot cheaper, usually a cheaper grade of opal is used, and the slices are a lot thinner than a solid stone. 3. A triplet. This is made of a very thin slice of opal, with a dark understone, and a clear overstone, sometimes glass is used. The opal layer is so thin that it needs that overstone to protect it. These are usually mass produced. There are also mosaic opals, which are such tiny pieces of opal that will not make a piece of jewelry alone, so many are glued into an acrylic base over a dark stone.
Opals are mined in several places in the world, the two most common are Australia and Mexico, although Peru is coming into the market with milky pink and blue shades of opal.
The most valuable opal in the market today is the Australian black opal. The term black can be deceptive, since they are graded on a scale of one to ten, with N1 being black, and N10 being white. So why are they called black? This term comes from the mines, rather than the stones. The black opal mines are mostly in the Lightning Ridge area of Australia, and most black opals come from there. Many lighter opals are mined there also, thence the N1 to N10 designation. A good black opal can cost upwards of $5,000 per carat! A basic black opal with not very good polish, perhaps some potch on the face, and only one refractive color, not well dispersed over the surface can be as little as $25.00 per carat.
Each opal is unique. Each good opal has a pattern and a refraction area that is unmatched in any other stone. The only way to get two matching stones is to go to white opals. You can get close, you can match the refractive colors, but they are not the same. This can be frustrating when trying to buy a pair of earrings, but it is wonderful when buying rings, pendants, etc., since no two will be alike, and no-one will ever have one exactly like yours. If you buy the same size topaz, amethyst, garnet, or any other faceted type stone, others will have an exact match of yours. Your opal will always be one of a kind!