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The Bielby Layer

There are two basic ways of polishing stones. The first is by abrasion, sanding with finer and finer grits until the remaining scratches are invisible to the naked eye (but still there). Faceted stones are polished this way, as are many cabochons.. Stones polished by abrasion are subject to staining and can be dyed if they are porous, as many stones are.

The second method is by polishing the stones on a disc made of leather, felt, etc., or synthetic materials having similar characteristics. So-called rare earths, such as cerium, tin and aluminum are applied in paste form to the material which is kept moist. The polishing action is quite different in this case. Intense heat is momentarily generated by friction where the stone contacts the disc and causes a layer of the stone, a few molecules deep, to turn to a fluid state. This molecular depth is called the Beilby layer after its discoverer. The flow produces a scratchless finish. Diamonds cannot be polished in this way because the temperatures generated are not high enough.

Via Ted Roberts, Skagit Gems