I was polishing an opal cabochon recently and the finished product just wasn’t coming out right. The polish looked OK with the naked eye, but under 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 .008” or a 6” X .12” diamond blade on my trim saw 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, 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.
Sanding: As a rule for progressive sanding steps, double the grit number for your next smoothest sanding abrasive. My sanding steps are: 320 sanding (or 280), 600 grit, 1200 grit and 3000 grit for pre-polish. 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.
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. Now I am using a Cab King with a canvas polishing pad and 14,000 diamond compound…works great as well…there are truly many, many polishing methods that work well. I recently bought an 8” 50,000 grit diamond disc for my large 8″ polishing unit to use after 14K diamond or tin/cerium…I like it…adds a bit of sparkle.
Contamination is a real danger here. The 50K grit disc is very easily contaminated. 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.