Recently I was polishing an opal cabochon and the finished product just wasn’t coming out right. The polish looked hazy with the naked eye and under 2X Optivisor magnification there were small flat spots and a few deep, smoothed out scratches. This isn’t bad for your personal work, but if you are selling custom jewelry projects or finished cabochons, the finish under magnification, is very important.
My next thought was “time to get back to basics”.
Inspection: Carefully inspect your opal before you begin cutting. Hold the stone alongside the edge of a strong light so that the light shines into the opal without shining into your eyes. Check for cracks and inclusions: especially sand pockets. I use a 6” X .12” diamond blade on my trim saws to cut around imperfections. It’s a good idea to have a spare blade as they kink easily. Don’t grind off material and waste it…you may need the small pieces for accent stones, inlay or chips for small glycerin filled vials. Continue checking for imperfections during work, as grinding may expose hidden cracks or pockets.
Grinding: Course 80 and 100 grit grinding wheels cut material very fast but leave DEEP scratches on opals, which may not be seen until you are into the polish stage. They also generate vibrations which may fracture your tender opal. Course wheels dig much deeper than a 220 or 320 grit wheel and more much material must be removed to smooth out these deep scratches. Opals are soft and a 220 or 320 grit-grinding wheel will suffice for course grinding. I prefer a 320 grit wheel for all opal roughing. And of course if you’re doing a lot of cutting you might want to look at sintered grinding wheels…they cost more but have many times the life of an inexpensive nickel plated wheel.
Sanding: As a general rule for progressive sanding steps, double the grit number for your next smoothest sanding abrasive…the higher the number the finer the grit. My sanding steps are: 320 sanding (or 280), 600 grit, 1200 grit and 3000 grit for the pre-polishing step. Cross contamination is a danger especially in the finer grits. Be especially careful of your 1200, 3000 grit wheels and wash your hands and stone before changing to a new belt. I mostly use diamond, but have found that a well worn 600 grit silicon carbide belt work and wears well and are inexpensive (still living in the dark ages, eh). And can be a great pre-polish belt. Final shaping done on too coarse of a sanding wheel may cause of flat spots. Or perhaps, if your unit has a motor driving the unit via pulleys, your unit may be running at too high an RPM.
Polishing: It is hard to teach old dog new tricks! I still use tin oxide with a little Linde A (submicron alumina) 10:1 ratio or cerium oxide slurry (what a debate, eh) on leather on one of my old cabbing units. But now I am mostly using a 6″ Cab King with a canvas polishing pad with 14,000 diamond compound…works great as well…there are truly many, many polishing methods that work well.
When polishing clean your stone and hands as though you are heading into surgery. Try not to touch polishing wheels when not in use and warn others. Keep spare leather disks in plastic bags and don’t be afraid to change them when you see scratches.
I hope this helps…thanks…Steve.