Glass Isn’t Always an Imitation

Since the Annual Tucson Gem and Mineral Show is this month, some of the more unusual things in the colored gem world are on my mind. I wanted to take the time to comment on a part of the jewelry industry that gets overlooked and has some of the coolest science behind it.

Faceted Moldavite

Faceted Moldavite

Man-made glass is a wonderful advent of technology. It has all kinds of practical uses from windscreens to containers to magnifying lenses. It has been used to make artwork and jewelry for thousands of years, and my showroom is full of examples. There was a time when glass was actually esteemed higher than many gem materials.

However, glass has had its more dubious uses. Over the last hundred years, as the cost of glass production has plummeted, glass has been used to make inexpensive costume jewelry. If it is ethically represented as what it is, glass is a cheap way to get a big look. But every industry has its unscrupulous characters, and glass has been used to deceitfully imitate more expensive gemstones like diamond, ruby, and emerald.

But it’s not always so. Nature has its own selection of glasses that are legitimate gemstones in their own right. There are several varieties of natural glass, but the basic formula for all of them is silicon dioxide.

Some of you probably recognize that particular chemical formula as quartz, and you’re not wrong. Although they share a chemical formula, quartz grows in a repeatable and predictable crystal habit, whereas glass is randomly formed and amorphous, lacking an organized crystalline structure. Glass is, essentially, melted and rapidly cooled (fused) quartz. Pure fused silicon dioxide is called “lechatelierite.”

There are three varieties of natural glass that are of interest to the art and jewelry industry: fulgurite, volcanic glass, and tektites.



Of the three, fulgurite is the closest to a pure lechatelierite formula. The word fulgurite comes from the Latin word “fulgur,” meaning “lightning,” because fulgurites are formed by a bolt of lightning striking silica sand. The strike creates a tube of molten silica down into the ground, which cools quickly enough to not redevelop a crystalline structure. Some of them can be several feet long. Fulgurites are most typically used as pieces of freeform art, or are fashioned into cups, chalices, bowls, and other art objects.

Volcanic glass, or obsidian, is molten silica that is discharged in lava flow during a volcanic eruption. When the lava flow reaches a body of water, it cools very quickly, forming glass. Volcanic glasses are usually chockfull of impurities since they are formed from molten rocks, and so they also come in several colors and distinct, attractive patterns. Obsidian is often fashioned into gemstones, or harvested in freeform as art objects.



Tektites are somewhat of a mystery. There are several varieties recognized around the world, but there’s no agreement of how they are formed. The hypothesis that is most widely accepted is that they were formed by meteoric impacts since they are found in or close to meteorite craters. The heat generated by the strike instantly melts any quartz in the vicinity, while the force of the strike flings the molten silica up into the air, where it cools too rapidly to realign its crystalline structure. Most tektites, like Moldavite and Libyan Desert Glass, are brown to bottle-green, but some are black, pale yellow, or even white. They have been cut as gemstones and make interesting conversation pieces.

So wear your fashion jewelry and enjoy the glitz and glamor of it all. Don’t, however, believe that all glass is used for is the fake stuff.

Posted in: Glass, Tucson Show

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