Tips For Polishing Synthetic Corundum


Synthetic or lab-grown corundum (ruby and sapphire) has the same physical and chemical properties as natural corundum. Polishing synthetic corundum can leave pits on the gemstones. Gem cutters can avoid this problem with the right tools and techniques. “Artificial Ruby Hemisphere Under a Monochromatic light” by D.328 is licensed under CC By-SA 3.0
Synthetic or lab-grown corundum (ruby and sapphire) has the same physical and chemical properties as natural corundum. Polishing synthetic corundum can leave pits on the gemstones. Gem cutters can avoid this problem with the right tools and techniques. “Artificial Ruby Hemisphere Under a Monochromatic light” by D.328 is licensed under CC By-SA 3.0

Question

I’m having a problem polishing synthetic corundum. I hope you can help. I’m cutting synthetic corundum, small 4 x 4 x 4 tri. The problem is I’m leaving pits in the stone after polishing.

I’ve tried almost everything, from tri-laps, ceramic laps, 3000 NuBond, to lucite w/8000 diamond. I’ve varied the speed, used oil, used water. I’ve cut with 325 NuBond and then polished. Cut with 600 NuBond and then polished. Cut with 600 dia. lap and then polished. Tri-lap 600, tri-lap 1200, same result.

The NuBond laps are well used. Could this be the problem. The best solutions I’ve found for polishing synthetic corundum are to cut ½ degree off of finish and then polish or polish on a corner with 3000 NuBond. This works somewhat, but it’s not great.

I have 18 of these stones left to cut and polish. I’d appreciate any advice.

Answer

Polishing gemstones can be a mystery. However, what you’re experiencing is a common problem with polishing synthetic corundum. The NuBond laps are a bit gentler than nickel bonded laps but they leave pits that are almost impossible to polish out. There is a way around this, though.

  1. Take either a ceramic or a cast iron lap.
  2. Give it two or three sprays of 14,000 diamond, then a shot of WD-40 or a few drops of oil.
  3. Mix it around with your finger but don’t wipe it off as you normally would. This creates a slurry that will pre-polish and polish all in one step.

I usually begin with a 600 finish from a nickel bonded lap. You can use something coarser in a NuBond. You’ll find it takes about as much time on the lap as two steps. With the slurry, however, you eliminate the pitting. Plus, it’s one of the few methods for polishing synthetic corundum that works for just about everyone who tries it!

Donald Clark CSM IMG

"Untitled," an artisan polishes gems during the reopening of the refurbished Turquoise Mountain Foundation in the heart of Kabul's Old City, Afghanistan, on Monday, May 09, 2011(Mireille Zieseniss/Department of State), by US Embassy Kabul Afghanistan is licensed under CC By-ND 2.0
“Untitled,” an artisan polishes gems during the reopening of the refurbished Turquoise Mountain Foundation in the heart of Kabul’s Old City, Afghanistan, on Monday, May 09, 2011, by US Embassy Kabul Afghanistan is licensed under CC By-ND 2.0