This is not "opal for dummies" cause anyone who loves opal is no dummy.
The foregoing is some information about opal and the trade that I have gleaned over the years. I do not claim to be an "expert" ( an "ex" is a has-been and a "spurt" is a drip under pressure). But 20 years working with opal and learning from those whov'e come before me has prompted me to share this at the request of my customers and friends. The following facts are regarding Australian opal, and are not mutually exclusive. There will always be exceptions to the rules with opal.

As each opal dig produces a different pattern and varying quality of gem, it would be safe to say that there are no "experts" in this business,only experienced dealers and cutters and even those of us who may claim the experience are subject to the ever changing rules of supply and demand, coupled with the suprise of meeting with a completely new and different material from day to day. However.....there are some basic constants that have evolved over the years that allow us to get a benchmark. This comes in 2 parts and follows the rule that rarity drives value. First is base color or the background color of the material (not the fire). Second is the fire( pattern,color mix and brightness). The hard part is that these 2 attributes will often cross over each other as I will explain.
If fire color mix and brightness are assumed to be equal across the board in varying base colors, then black is first in value,crystal (water clear base) is a close second, grey or semi-black next, then jelly(translucent like jello) and white base or "light opal" last. Boulder falls into the dark opal or semi-black range because of the darker background.
Fire colors are also in order of rarity and here it gets a little and orange are most rare but ruby red is most prized, then azure or deep blue and green last. Pastels such as pink, light blue, light green, are at the bottom due to low brightness. Most opal is multi-colored and it is the dominating color that sets the value here.
Here's the cross opal can be worth less than white base if the black has only one color of very weak fire and the white has brilliant, intense color(one or more). Some will disagree with me on this and say that the black always has more value because it's a black opal. I like asparagus but not brussel sprouts, oh well. Anyway, this logic applies to any combo of base colors but in general, darker background is more prized than the lighter because it kicks the color out and crystal is in a league of it's own, seldom being dull and very rare at the mines.
Market grading usually falls into 3 grades, commercial grade, gem grade and top gem. Commercial grade can be low commercial or high commercial. Low commercial requires direct lighting to see any flash of color, will almost always be white base(sometimes low end blue jelly) and should sell for about $25 to $50 per oz. rough.When taken out of direct light i.e. sunlight, halogen or a 100 watt bulb etc., the color dies. High commercial will retain maybe half  to three quarters of it's color or brightness when taken out of direct light and should sell for about $60 to $200 per oz. rough. Note: white base, due to it's light background, almost always requires direct light to look it's best and therefore seldom makes it above high commercial but it does happen.
Gem grade will retain most if not all it's color when out of direct light,maybe not quite as bright but the full pattern is still there. Gem grade will sell anywhere from $300 an oz. for white base to $1500 per oz. and up for grey base, dark opal or semi-clear bands in a base material. Most "good" opal falls into this catagory and some dark base opal such as Lightning Ridge material will actually look better in lower light. The Aussies call this a "shade stone". That's why Ridge material is among my favorites; I advertise it as "glowing in candle light" and it does! So does good crystal whether it's solid, a layer in boulder or a doublet or triplet. (Most top grade triplets are made with crystal or semi-crystal(better than jelly) and the rough used can cost the manufacturer $5,000 per oz. and up.
Top gem ,when out of direct light, will either keep all of it's color and 80%  or more of it's brightness, or, get brighter, or, do a color shift where the colors actually change hues due to the change in frequency of light. This is called pleochroism or the stone is said to be pleochroic. I once saw some crystal that under a small high intensity light was international orange and when taken away from the light, turned violet. Good crystal and blacks with good color automatically fall into the top gem realm at the high end.Cost? well, you may find a small piece in a $200 or more ounce of rough but if it's consistent through the lot, the price would be $1000 per oz. and up.
Now we come to parcel grading. It used to be that the miners did not grade their parcels very carefully and you could get some really great deals.Today, as more restrictions are put on mining operations and the overhead goes up, the miners are sorting their lots closely, usually leaving a few "king" stones to sweeten the pot. (wer'e talking commercial to gem grade here, when your'e paying for top gem or high end gem grade, the more you spend, the more consistent is the parcel). Most larger wholesalers buy their rough in multiple kilo quantities. When your'e buying that much opal your'e going to get a spread of quality in enough quantity to grade it again and still have decent lots for sale and that's just what they do.How many times they "cut it" determines how good a mix you get in the ounce. If I buy 5 or 6 ounces of gem grade for resale and it's not a very consistent lot, two things are true, it isn't high end material and there aren't enough stones to grade it again.(when your'e buying rough at $50 to $200 an ounce, there's almost always high and low in a lot, some stones worth more than the oz. price and some worth less, with the majority on spot, (or at least that’s the way it should be). However, if I buy a kilo (2.2 lbs.) or more, there's usually enough stones at the high and low end to split the lot into 2 grades, without grading too closely. I like consistency but it's not always possible to obtain, specially at the lower price levels. The miner worked just as hard to get that chunk of potch as he did for that bright stone next to it and he wants to sell it all so we wind up with mixed grades in a parcel and if the price is right, it's well worth it.
Here are a few facts that concern buying and cutting and may answer some of the more commonly asked questions:
1) Rough opal is sold by weight...either grams (there are 5 carats in a gram), troy ounces (31.1 grams per oz. but most of us round it to 31 grams) or Kilograms (1,000 grams which is 32.15 troy ounces or about 2.2 pounds U.S. Whenever I see rough for sale by the gram, I calculate what the ounce price is..this tells me if I'm getting a good deal or not...some dealers will sell you a good looking piece at lets say, $20 a gram, well, thats $620 an ounce..better be darn good stuff for careful.
2) Cut stones are sold by the carat (there are 155.5 carats in a troy ounce) If you see rough being sold by the carat, it had better be very clean material...rubbed (all or most of the waste ground off) or trimmed to the color.
3) The term "rough cut" is a contradiction, it's either rough or cut, can't be both.
4) Potch and color is material that is potch (opal without color) that has lines or viens of color running in it as opposed to what's called "skin to skin" (color from one side to the other).
5) "Mine run" opal is low end material that's left after the miner pulls out the "good stuff". It's called mine run because 90% of what comes out of the ground is the low end.
6) Last but not least, a word about the real value of the rough you just bought to cut...most rough is potch and color which means about 50% is going to be you bought one ounce for $100 and there are 155 carats in the ounce, your'e paying about 65 cents per carat. With 50% going to dust on the wheel, your'e real price is doubled to $1.30 per carat cut. Commercial grade white base (cut stones) will wholesale (what the average jeweler will pay) at about $12 per carat....youv'e just made 900% on your money. Try running the waste factor and carat cost on that next piece of gem grade dark base your'e cutting (chances are it's worth at least $40 a carat) and if you don't have a carat scale, get one and stop cheating yourself. Good opal is like real estate, theyr'e not making any more of it and the supply is going down while the price goes up..that's why it's called "precious" opal....Good cutting!
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