Question:
I deal in vintage costume jewelry. I just picked up three crystal necklaces
at a flea market. I suspect that one of them might be rock crystal
(quartz?) instead of leaded glass. I suspect this because it is noticibly
colder to the touch than the other necklaces. Am I on the wrong track, or
is this an indication of composition?
Also, I just sold a necklace on ebay which I thought was orange crystal.
The buyer, who is a jewelry dealer of some sort, emailed me to thank me, and
told me the necklace was actually real cut citrine. Also, I sometimes get
sterling jewelry set with stones that I suspect to be semi-precious.
Short of a class at the gemnology isntitute (which I have been considering,
btw), is there any simple way to tell semi precious stones from glass?
Answer:
-Actually, no. An RI is the most informative test to determing exactly which type
of material OTHER than glass you might have, but a refractive index measurement
won't always tell you if you have glass, since glass can have almost any
refractive index. Of course, if you see a double refractive RI measurement,
then of course, it can't be glass. But a polariscope is simpler to use for this
purpose, and you can make a home brew version of one for a few dollars, while a
refractometer is harder to use, and will cost a bunch of money. And the
appearance of glass or plastic under a polariscope is different, usually, from
that of other single refractive materials.
Frankly, with just a little practice, all you really need, most of the time, is
just a good loupe, to seperate glass from other materials in costume quality
materials. There are, of course, exceptions, but they are less common.
With just a loupe you can look for:
Conchoidal fracture/chips
Cutting/polishing style and quality
Mold marks
gas bubbles
foil backing
curved "swirl" lines
inclusions not consistent with glass, such as feathers, healed stuff,
fingerprings, crystal inclusions, liquid or multiphase inclusions, and the
like, or color zoning/banding
dichroism (often somewhat detectable without a dichroscope in double refractive
gems)
Between these simple observations, and the context of the "stone" (mounting
style and technique, etc), it's rare to not be able to determine if a stone is
glass or not...
