How opal is graded and sold

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. Colour 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.

Mine Run. Direct from the mine. The stones have not been cut or ground down. This means that there is more guess work in the cutting. Purchasing mine run opal can be risky if you are not very experienced.

Off Cuts. The miner has removed whatever opal he has a market for and sells you what's left. With off cuts, you can usually tell what you are going to be able to cut. You must still watch for cracks in the opal because once a crack becomes obvious, a stone can lose half its value.

Rubs. This can often be the best way of buying rough if you are not very experienced. The miner has cut and ground the stones into basic shapes, after having removed most of the rubbish. What you have left is the stone nearly ready for the dopping and polishing process. You have the satisfaction of cutting your own stones without the high risk of buying mine run rough.

I've put together detailed opal cutting instructions in my CDROM, Opal for the Ordinary Bloke. My CDROM should be available soon, but here's a few excerpts to help you in the meantime.

Opal is one of the easiest stones to cut. The trick is chasing the colour to make sure you approach the colour 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.

Your first task is to search an opal for any hint of colour. Then, you use a wet grindstone to grind off the potch until you can see where the colour is going. When you've exposed enough colour, you can choose the top and bottom for your stone based on where you see the best colour contrasted against the background colour.

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. 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. After you've cut the stone into the basic shape, use a wet grinder to finish shaping and remove rough edges.

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 matt polish. 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.) *potch is unformed opal