Polishing Gemstones

The hardness, index of refraction, and transparency of gemstones determine the way they are cut and polished. Since ancient times gemstones have been shaped in two main styles (1) as faceted gems, with many plane polished sides called facets, or (2) as nonfaceted, rounded, and polished gems, known as cabochons. 

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Agates started to be polished in Idar-Oberstein in the 1800s and the city is now a center for polishing gemstones. Antwerp and Amsterdam have been the centers for... 

Natural resins have been collected by hand throughout recorded history and used with minimal processing. They are reported to have been used in the arts, both in paints and for polishing sculptures, as early as 350 BC. Amber, the hardest of these resins, has been used as a gemstone from early Greek history to modem times. The electrical properties of amber were first recorded about 300 BC. Following is a description of commercial natural resins that are available in the United States. 

Cutting, grinding, and shaping stone, and in particular burnishing and polishing the surface of stone as well as metals, requires the use of abrasive materials that are harder than the solids to be cut, ground, burnished, or polished. Sapphire and ruby, two very hard gemstones, for example, can be cut or polished only with the assistance of diamond powder, an abrasive that is harder than sapphire or ruby. Diamond is the hardest material... 

Zirconium silicate (ZrSiO ) is one form of the mineral whose crystals when polished are known as cubic zircons, which resemble diamond gemstones. 

Jet is actually just a very hard and dense kind of lignite coal. It was probably plant material millions of years ago that has become fossilized and blackened over time. It often comes from northeast England, where it is derived from fossil driftwood buried under the sea. Its primary drawback as a gemstone is that it will burn (since it is basically just highly polished coal), see also Glass Minerals. 

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