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August 1999

Gemstones & Pearls:Gemology

The Great Pretender

A glass imitation of peridot can fool anyone, including a noted cutter

Instructors at major gemological schools are prone to caution that glass can look like anything. Not only does it visually simulate other gems and natural glass such as obsidian, its physical properties can be manipulated to overlap those of the other materials, opening the door to deception.

Consider the recent dilemma of one of America's most noted gemstone cutters, Arthur Anderson of Carrboro, NC. At a regional show in Raleigh, NC, Anderson came across a dealer who showed an impressive collection labeled Chinese peridot.

"The price was extremely low for the cut stones, which looked like Pakistani peridots, so I bought all of them," says Anderson. He intended to use the faceted gems as "preform" material for his own creative cutting. "The material looked like it would cut to $100-$200 per carat," he says.

At the polishing wheel, Anderson made a sinister discovery. The material wouldn't polish properly using the standard grit for peridot: tin lead with Linde "A." "Peridot is not my favorite gem to cut because it's temperamental and has different hardness and directions suitable for cutting," he says "But this was really different."

The material polished nicely with cerium oxide – the industry standard abrasive used in polishing quartz, but quartz doesn't come in green.

Anderson shipped the stone to the Gemological Institute of America. GIA found it to be leaded glass.

Anderson alerted other cutters and gem dealers to the peridot impostor and arranged for a refund from the dealer who sold him the stones. "The main point is that I could have passed on this material unknowingly," says Anderson. "This is an industry where reputations can be damaged just like that. One strike and you're out."

You can distinguish peridot from glass through the following tests:

  • Check any suspect material under a microscope. Round or doughnut-shaped bubbles are a clue the material is glass. Glass may exhibit internal whirling flow lines. Pitted surface features, especially small rounded depressions, suggest glass polished at an internal gas bubble's level.
  • Dark inclusions (which can be chromite crystals) and disk-shaped internal fissures (known as lily pad inclusions) suggest you have peridot.
  • A few other tests (refractive index and specific gravity) should confirm initial findings. The refractive index of peridot is 1.654-1.690 with a birefringence of 0.36. The refractive index of glass can be manipulated with lead content and coloring agents. It ranges between 1.470 and 1.700, which can cause confusion, but glass is amorphous (it has no crystal structure) and not birefringent. Glass can fool some gemologists with an optical anomaly that makes it appear doubly refractive.
  • Consider that manufactured glass can be confused with natural glasses such as obsidians, moldavite and tektites.

The stone on the left was fashioned from a marble to show typical glass inclusions – swirl lines and doughnut-shaped gas bubbles. The stone on the right is also made of glass but shows no such clues. It was unknowingly purchased as "fine peridot" and recut by Arthur Anderson of Carrboro, NC. Anderson became suspicious when the stone failed to polish properly using a grit designed for peridot. Stone on left courtesy of Robert Weldon, G.G. Stone on right courtesy of Arthur Anderson and Gem Reflections, San Anselmo, CA.

– by Robert Weldon, G.G.



Copyright © 1999 by Bond Communications.



 

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