synthetic star corundum boulessynthetic star corundum boules

Tips for Polishing Synthetic Corundum

Polishing synthetic corundum without leaving pits requires certain techniques and tools. Get expert lapidary advice here.

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Question:I'm having a problem polishing synthetic corundum, small 4 x 4 x 4 mm triangles. After polishing, I'm leaving pits in the stones. I've tried almost everything, from tri-laps, ceramic laps, 3,000 NuBond, to lucite w/8,000 diamond. Same result. 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 diamond lap and then polished. Tri-lap 600, tri-lap 1,200, 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 3,000 NuBond. This works somewhat, but it's not great. 
synthetic star corundum boules
Synthetic star corundum boules. Photo © Joel E. Arem, PhD, FGA. Used with permission.

Answer: Synthetic or lab-grown corundum gems (rubies and sapphires) have 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.

Preparing Laps for Polishing Synthetic Corundum

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 grit 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

Synthetic corundum
Artificial ruby hemisphere under a monochromatic light. Photo by D.328. Licensed under CC By-SA 3.0.

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