Polishing Charoite

Charoite is purple calcium potassium silicate. It is usually found combined with minerals of other colors, including black (Augerine Augite), orange (Tinaksite), and transparent crystals (Microcline Feldspar). Most Charoite comes from the countries of the former Soviet Union.

I have not found it to be very heat-sensitive. I hot-dop it, dry-sand it, and put it in the freezer to remove the wax with no problems so far. I think people have misattributed its known propensity to fracture to heat. I think they were working a stone and when it broke, they tried again and had better luck with more water and a lighter touch. I think the lighter touch was the magic, not the water. Charoite can “burn” when grinding or sanding, but I can do the same thing to agate if I hurry too much.

I have found that the white banding between the purple areas is softer than the rest, and the stone is subject to fracturing along these lines. This usually occurs when grinding, but I have had it happen during sanding, and only rarely during polishing. If it has held together long enough to get to the polishing stage it will usually be okay. I do my pre-forming on a fine grinder (380 grit) using a light touch and hoping for the best.

I have read that charoite can only be worked with diamond but I have not found this to be true. I find I get the best results treating it like jade. After sanding through 1200 grit with diamond (although I don’t think the stone would care if I used silicon carbide for this), I dry-sand it using 1200 grit silicon carbide, then polish it with aluminum oxide. I find it easier to control the under-cutting with silicon carbide than diamond, but I know others prefer diamond. Any technique that keeps stones from undercutting during sanding should work with charoite.

I have used cerium oxide, diamond, and aluminum oxide to polish charoite. While all three worked, I like aluminum oxide better. Aluminum oxide is available in a variety of grits, is convenient to use, has a hardness of 9 on the Mohs scale, and is relatively inexpensive. For charoite, I like to use a two-step polishing process with this abrasive. I usually start with the 1.0 to .35 micron powder mixed into a paste with water or vinegar, and I finish with the 0.2 or 0.1 micron type, prepared the same way. I think the aluminum oxide works as well as—or better than—diamond abrasive for most stones under a 9.0 hardness.

by Dick Friesen