The unique characteristics of Ethiopian opal make cutting this material more challenging than Australian opal. It is volcanic in origin, and the rough pieces often have hard packed sand pitted in the stone, and/or odd contours containing deep concavities. These features lead to the rough having a relatively low yield – typically around 20%.
The first step in cutting this material is grinding off all the sand spots and, unless you are carving the rough, removing the concavities. This can sometimes be quite frustrating, as occasionally the sand pit may extend several millimeters into the stone, or you have to cut the stone into smaller pieces if a concavity cuts deep through the stone.
It takes longer to cut this material, too, since you usually can’t cut it to completion in one sitting because you can’t tell how to orient the stone once it is hydrated and the color has disappeared. I’ll rough out the stone one day, then let it sit for a few days for the color to reappear before deciding how to orient it and finishing the cutting. The color in Ethiopian opal often goes through the entire stone, rather than forming in a single layer like much Australian opal, and can present a challenge as to how the stone should be oriented. I’ve had plenty of stones where I begin to finish the back and realize that the back looks nicer than the front! I’ll round these off and take them to a final polish, resulting in a beautiful, double-sided stone.
I initially shape the rough using a #220 diamond wheel, and then smooth the stone with a series of Nova wheels, using plenty of water throughout the process. A progression through a #280, #600, #1,200, and #3,000 Nova wheel results in a stone with a smooth, continuous surface free of scratches or flat spots. Final polishing is done with French cerium oxide on a damp leather wheel.
Although these opals have demonstrated excellent stability once cut, when they FIRST hydrate, some of them develop cracks quite rapidly that extend through the entire stone. I just “let it go” and saturate completely, and then simply break the stone apart along the crack lines. It is a major bummer, but unfortunately part of the process, when that big awesome piece of rough now ends up as 2 or 3 smaller pieces! Sometimes, small cracks may develop as the stone begins its initial drying out process. Either way, it is important when cutting this opal to truly “beat it up” during the process to ferret out any material prone to cracking. You’d much rather have the stone reveal its weakness during the cutting process rather than once it’s ready to be set in a nice piece of jewelry! My opinion is that it’s crazy to cut this material dry, because you don’t find out the stone’s weakness until it is hydrated!! Although some of the material does crack during the cutting process, my personal experience is that once the stone has been polished and gone through the saturation/drying out process a few times without developing any cracks – you’re home free and have a stable stone. I have cut hundreds of these opals and have not had a single one crack or craze ONCE IT HAS BEEN CUT AND POLISHED!!