Last I heard, opal is made from tiny little spheres of cristobalite, which is a form of quartz only in one or two crystal systems other than the one quartz is in. However, I don’t understand how this explains why opal looks like it does, with all those colors.
2) There is an orange and other shades of orange to pure red kind of opal. This kind is somewhat clear or clear but a little strangely cloudy and it does catch the light and shine forth! But what explains this look, and since I’m seeing so much of this material at the shows, are they manufacturing it?
3) I saw a beauteous ring at an antique show that had a
big opal in it that had all colors in it; the left side of the opal was darker than the right but it didn’t seem to be two opals in the ring, and there waas a lot of green in the brown besides having all sparkly colors. It was set in platinum with a design of a ribbon-like band of small little diamonds, and the ring part of the ring was gold. They had reduced it to 1800 bucks from 3000. However, my friend asked the man is this a boulder opal or a black opal and he shrugged and according to her became evasive. She says boulder opals are cheaper than black opals. Also, I’ve seen this beautiful ring for years, evidently he never sells it.
What is this ring really worth?
From the Manual of Mineralogy, 21st ed. by Cornelius Klein and Cornelius Hurlbut, Jr. (1993, John Wiley and Sons, Inc.), pages 266-268.
Argh, Pantellerite, you just gave me flashback to mineralogy lab. ::shudder:: 
don willard, have a look at this page for a discussion about determining the value of a particular opal.
There are four main factors involved in determining the value of an opal: type of opal (e.g., solid, boulder, matrix, assembled, synthetic), base color, brightness of color, and weight. When I was in Australia, I saw a number of pamphlets and such explaining valuation of the stones. (Aussie laws are pretty strict regarding the descriptions of opals to prevent folks from getting ripped off by unscrupulous types, but no such laws exist in the U.S.) If I remember correctly, the more valuable opals are solid, have more red or orange (they are “fiery”), and of course are properly cut. High-quality solid “black” opals (not really black, but very colorful with no white) are not that common, and thus would be more expensive than less colorful or common white varieties.
Lots of opal jewelry is made with doublets or triplets, i.e., the gemstone isn’t solid opal. In a doublet, a whitish stone may be backed with a flat, dark backing so that it appears more colorful. Triplets are often made from thin slivers of boulder or matrix opal sandwiched between a flat dark backing and a rounded glass top, so that the stone appears bigger. These can really be quite pretty, but you should pay considerably less for such jewelry.
If the dealer has had this ring for years, it is likely because the piece is waaaayyy overpriced for the quality of the stone. The fact that he became evasive when asked about the stone suggests, unfortunately, that he might be trying to pass off a doublet or a triplet as a solid stone to some unsuspecting person. I would steer clear of him.
There’s something ironic in that item 3 on Don’s original list wasn’t “Hi, Opal!”
Sorry…nothing sensible to add.