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Hints & Tips

Working with Opal

How Opal is Graded

Rough opal parcels are sorted into three grades: tops, middle, and low. Each tops parcel has a King Stone, which is the best stone in the parcel. Some parcels have several King Stones. Color is the primary criteria for grading, but the graders also take into consideration the number of imperfections and faults, and whether a stone is the right shape to be cut into an oval or one of the other popular shapes.

You Can Buy Rough Opal in Several Different Conditions

You have the satisfaction of cutting your own stones without the high risk of buying mine run rough. Opal is officially sold in troy oz. There 20 d.w.t. (dead weight ton) pennyweights to a troy oz. There are approximately 30 grams to a troy oz. and approximately 30 troy oz to a kilo.

General Hints and Tips

  1. Not all opal is valuable. Potch, sometimes called "common opal" does not reflect colors. Potch does have its use, however, as a backing for doublets and triplets. When there is color mixed with potch it's called "potch and color."
  2. Some classes of rough opal are unsuitable for making gemstones. Some are very porous. Others have a chalky appearance. Still others can lose their color in a short period of time. And yes, it is common for certain types of opals to shatter in a dry atmosphere when the water from the gel evaporates. The best protection for buyers is to purchase opal from a knowledgeable and reputable dealer who accurately details exactly what you are purchasing and is willing to guarantee the stones. Note: If you buy rough opal, there is no guarantee that it will not crack. This is the calculated risk you take and is part of the "excitement" of opal cutting.
  3. When examining opal jewelry, look at the stone from the side to determine whether it's a triplet. Solid and doublet opal domes are simply the opal formed into a curved dome shape. Triplets have clear crystal domes to protect the flat opal surface.
  4. It is very difficult to tell whether a stone is a doublet or a solid if it is in a setting. It helps if the back of the setting is open, but it is still no real indication because the back of a doublet and a solid looks very similar, although a doublet is usually perfectly black on the back whereas a solid black opal usually has some imperfections. So if the back of a stone looks too perfect, you have to be a little suspicious. For this reason it's best to buy opal from reputable dealers who know what they are doing because, unfortunately, many jewelers also are not experienced in opal.
  5. When buying opal over the Internet, ask the seller to give you a summary of how close the graphic sample is to the real thing. Opal is very difficult to photograph and the appearance on the computer screen may give it a different look. Particularly this is the case in regards to size. Graphics come out in all different sizes, usually a lot larger than actual...so make sure you get your ruler out and actually draw the size on a piece of paper so that you will know in your own mind's eye the actual size of the stone.
  6. The same thing applies to the color of the stone. Make sure you get the supplier togive a commentary on the color so that you will know pretty well what you are getting. The camera sometimes picks up colors that are only minimal, while ignoring the feature colors.

Cutting, Grinding, Sanding and Polishing Tips

  1. Opal is one of the easiest stones to cut. The trick is chasing the color to make sure you approach the color from the right direction. Lapidary clubs are a good resource for new opal cutters. Ask experienced members for advice on the basics of cabochon cutting.
  2. Your first task is to search an opal for any hint of color. Then, you use a wet grindstone to grind off the potch until you can see where the color is going. When you've exposed enough color, you can choose the top and bottom for your stone based on where you see the best color contrasted against the background color.
  3. Let the stone dry then check for cracks and imperfections. Everyone wants to cut a big stone, but two or three smaller perfect stones are a better choice than one big stone with imperfections. Remember too, if you're planning to make a jewelry piece, settings come in standard, calibrated sizes. Unless you'll be making a completely custom setting, you're better off sticking with a common size.
  4. A diamond blade, 10 thousandths thick is a good choice for cutting an opal stone to size. Many lapidaries will tell you to run the diamond blade slowly with opal. This is a big mistake. Opal is not a hard material like agate, for example. Opal slicing is best done either in water or with water passing over it at the speed achieved from an ordinary 1440 RPM*, 1/4 H.P. motor with a .6" pulley on the motor and a 1 & 1/2 pulley on the shaft. If you don't force the stone and you have plenty of water passing over it, you will not cause any damage. In fact your diamond blades will last longer because they will not buckle as easy.
  5. After you've cut the stone into the basic shape, use a wet grinder to finish shaping and remove rough edges.
  6. The final step is sanding. Begin sanding with a 320 grit paper. If the paper is new, you may rub it on some potch** or a sheet of glass to dull its sharpness a bit. You can then use either a 600, 700, or 800 grit wet and dry sandpaper to gradually take out the finer scratches and bring up a matte polish.
  7. Finally, on a leather lap, use a paste of cerium or tin oxide (mix with water) to bring up the final polish. Your finished opal should have a nice, high dome for easy mounting. Save the chips! You can use those bits of opal to create stunning mosaic patterns. If you have a piece of opal jewelry that has become scratched or dull, just use the finer papers (700 or 800 grit). You can try to work the above process by hand by cutting small strips of the sandpaper and just rubbing them back and forth across the stone until the scratches are taken out. If the paper is too harsh, just tone it down by rubbing it on a piece of glass.

Next step is to get a piece of old sheet or pillow slip (cloth) and vigorously polish with tin oxide or cerium oxide. If you want to polish the gold or silver at the same time, use some metal polishing paste. Most of these items can be purchased from the hardware store, with the exception of the oxides which may only be available at Lapidary supply stores or at workshops that polish gravestones. (Stone masons) (Check in your yellow or pink pages on the telephone directory for locations.)

* RPM is based on 50 Hz cycles. Most (if not all) U.S. Electrical sources are 60 Hz cycles. Pulley size should be adjusted accordingly.
** potch is unformed opal

(Editors Note: Potch is a term that has had several different meanings over the years. At times it has been used to describe the matrix in which one finds opal, and also to describe common opal (opal w/o fire or play of light). Common opal is now sometimes referred to as gem opal if it has excellent lapidary qualities such as translucency and deep rich color, differentiating it from precious opal that has fire or play of light.)

Via Golden Spike Gem & Mineral Society - July 2006, Rock and Chips 8/06, Gems of the Rogue 10/06