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
"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,"
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
by Robert Weldon, G.G.
Copyright © 1999 by Bond Communications.