Polishing Gemstones for Beginners: Pre-Polishing Advice

Polishing gemstones is difficult to master. Pre-polishing plays a major role in how well a gem can take a polish. Avoid scratches with these techniques.

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Question: I'm looking for general information on polishing gemstones. I just can't seem to get a decent polish. I'm a beginning faceter and I'm using the Facetron. I've been working with a tin lap with 50k diamond powder with Graves extender fluid. I seem to get fine scratches no matter how clean the lap is. I really have to press hard and polish for several hours to even get a half-decent polish. The facets will shine well, but you can still see quite a bit of scratches under 10x magnification.

Some faceters have given me very helpful information on polishing gemstones. Several said they use a ceramic lap with 50k, then finish with 100k on the tin lap. I washed my tin lap and got a ceramic lap, but things only seemed to get worse. The ceramic lap really produced scratches on a piece of amethyst I'd been working on. The tin lap with 100k was better but still left quite a few scratches.

I'm not sure if I'm using too much diamond powder or too little. I'm not soaking my lap. I use just a couple of dabs from the bottle and work them around good before I start polishing. I'm getting pretty good at the cutting process, but polishing gemstones is really getting me frustrated. Any advice will be greatly appreciated.

Faceted amethyst gemstones. Photo  by Eden, Janine and Jim. Licensed under CC By 2.0.

Answer: Polishing gemstones can be one of the most difficult stages of faceting. Pre-polishing plays a major role in how well a gem can take a polish.

Pre-Polishing is the Key to Polishing Gemstones

The fault probably lies with your pre-polishing and not with your polishing. Scratches will inevitably show up regardless of your gemstone polishing technique if you haven't successfully pre-polished away all damage left over from the coarse grinding. You'd be surprised just how much gem material you must remove to eliminate the "healed" cracks left over from a grinding lap that perhaps was excessively coarse.

Ceramic laps are highly effective… and highly controversial. Eastern, a now defunct company in Michigan, realized that the extremely hard compound used to make ceramic laps produced scratching. (This material was originally designed to polish computer chips). This company re-formulated the compound into a softer and more porous personality, so diamond could nest in the divots. Faceters soon realized superb results polishing gemstones. Only the Graves Company sells ceramic laps made with the Eastern formula. Other harder laps don't work as well or as reliably as the Eastern-Graves lap. (Diamond grit rolls and scratches on the surface of other laps, whereas the diamond particles are nested in place in the Eastern-Graves).

Get a good book on pre-polishing. I highly recommend my own book, Master Gemcutting Tips. You'll find that pre-polishing is actually the key to polishing gemstones well, not the polishing itself, which is rather routine.

I hope this helps.

Gerald Wykoff, CSM GG

Dressing Your Ceramic Lap Properly Before Polishing Gemstones

The problem that you're having with a ceramic lap might be caused by not dressing your lap properly before you use it for polishing. A ceramic lap is needed in most cases for polishing gemstones that have a hardness of over 8.5 to 9. A ceramic lap is great for ensuring a nice flat facet on your material.

Step-by-Step Guide

To get a well-dressed ceramic lap that will keep any small scratches from appearing, you should try the following:

  • Apply the polishing grit on the lap.
  • Take a piece of synthetic corundum and put a window in it with 600 grit diamond.
  • Polish out this window completely on your lap.
  • Repeat this process using all of the lap at least five times. (There is an old saying about ceramic laps: if you haven't worked on the lap for at least four hours it ain't gonna work. Seems to be the truth).
  • Apply your polishing agent to the lap each time you start over.
  • After the repeat polishings, wash the lap with soap, hot water, and a plastic scrub pad.
  • After cleaning, window the synthetic corundum with 3,000 finish then polish this out. (At this stage, you will only need a small amount of polishing agent, if any, to do this).
  • Check the synthetic corundum. Small scratches on the lap itself will cause small scratches on the finish of your facet. If you see scratches on either one, start the process over.

This process works even on old ceramic laps that have started to scratch. I know this seems excessive and time-consuming, but it really works. If this dressing procedure is done correctly, the amount of polishing agent you need to use is kept to a minimum and the finish will be all that you desire. Finally, store your ceramic lap carefully to avoid scratching.

Ron Campbell, Central Coast Gem Lab

Ceramic Laps May Not be a Good Choice for a Beginner

Any time there is a problem with polishing, one needs to look at the pre-polish. If you are not getting an adequate pre-polish, you could be seeing scratches that are left over from the coarse stage. Or, you could have a contaminated pre-polishing lap which is leaving deep scratches.

You say the lap is clean. I wonder about your diamond grit. Make sure it's completely spread out in the extender fluid. No lumps! If that isn't the problem, then maybe your diamond is contaminated. Try another batch from a different supplier and, of course, make sure the machine is clean before trying it.

You shouldn't have to press hard while polishing gemstones. In fact, you should be holding the stone just barely against the lap. Too much pressure could cause the scratches. However, it doesn't explain why you aren't getting a good polish. It doesn't take much diamond on a tin lap, but I know people who really load it up and that works too.

Master the Tin Lap

Ceramics are really cantankerous. They always scratch quartz like your amethyst stone. Give up on ceramic until you master the tin lap. Here are some suggestions:

  • Read this article on laps and techniques for polishing gemstones again to see if there is anything you have overlooked.
  • Try a 3,000 pre-polish.
  • Use very gentle pressure at all stages.
  • You can't go directly from the coarse lap to the 3,000, so use a 600 or 1,200 in-between.
  • Try some new diamond bort. (Your current pre-polish lap could be contaminated).
  • Pay extra attention to cleanliness.

I hope this helps. I know how problems with polishing gemstones can take the fun out of faceting. I've been there!

Best wishes,

Donald Clark, CSM IMG

Gemstone Polishing in Wonosobo, Indonesia
Gemstone polishing in Wonosobo, Indonesia. Photo by Paul Arps. Licensed under CC By 2.0.

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