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The Beilby Layer

What is the Beilby layer? It is a phenomenon that brings about a polished surface. Sir George Beilby discovered that during polishing the surface of gemstones actually melted and flowed as a "glassy" layer over very fine scratches. He proved it by noting a certain scratch pattern, polishing the surface, and then recovering the scratch pattern by etching away the polished surface with acids.

In 1937, a Mr. Finch, using another technique, confirmed this finding. He reported that there were two types of polish: the Beilby flow and the surface that has such fine scratches that it appeared polished. The latter existed on those materials that were unable to flow in the Beilby manner.

The Beilby layer can occur in three ways. First as an amorphous layer much like glass, e.g., the polish on zircon and spinel.

Secondly, as an amorphous layer, but parallel to crystal planes and crystallizing again in these lines, e.g., calcite. In the third case, the layer forms by flowing but immediately crystallizes identically to the underlying material, e.g. quartz.

Distinguishing properties of the Beilby layer: It is very thin and usually slightly harder than the underlying material, probably due to packing of molecules by pressure.

There still remains some controversy over the existence of the Beilby Layer.

Some argue that the "flow" is not true melting, but rather a migration of molecules under pressure. Polish seems to be the result of a combination of temperature, polishing agent, and pressure, and varies from material to material.

But does is really matter, as long as we get a good polish?

Author unknown, from Geolap News, Aug. 1997