I don’t think the answer to this question is necessarily buying a
There are many ways to ID and separate gemstones and many are
cheaper and faster than using a refractometer. What is needed is some
gemological knowledge, as some have already stated. Depending on the
question asked, a little reading may be enough. For example,
identifying iolite should be easy enough. I don’t believe there is a
synthetic iolite, and I don’t think there is any other stone with the
specific dichroism which iolite exhibits. Looking at the iolite from
all angles, one should see a blue, a violet blue and a straw yellow
axis. I don’t believe any other stone shows this phenomenon.
As far as Tina’s other specific questions, there was a recent thread
on IDing natural vs synthetic amethyst which she should read.
Basically a refractometer will tell you nothing here, as synthetic
and natural have the same RI and birefringence and dispersion.
Magnification might show telltale inclusions of natural or synthetic.
Twinning suggests natural, as shown by the polariscope, but there are
now some synthetics with twinning. If no IDing inclusions are seen,
there is no cost effective way to tell natural from synthetic, as the
lab tests require sophisticated equipment and are very expensive.
Peridot is pretty easily identified by it’s dichroism and doubling
of back facets under magnification. It could be confused with green
zircon, but a Chelsea filter will differentiate the two.
Blue topaz is distinguished from blue zircon by the zircon’s doubled
back facets and from aquamarine by the dichroscope. From glass by
bubbles in glass. Blue synthetic spinel shows no dichroism.
The rhodolite garnet may show needles and other inclusions, will be
inert under the Chelsea filter, and may show anomalous double
refraction in a polariscope. A glass imitation (of garnet or any
other crystal) will be warm to the tongue compared to a crystalline
stone. Also garnet has more heft than most glasses.
The problem with the “buy a refractometer” response is that it does
not identify anything with an RI over 1.81, which encompasses several
synthetics, and it does not distinguish between synthetics and their
natural counterparts in most cases. GIA refractometers are not cheap,
and a good loupe with a dark light source, along with a Chelsea
filter, dichroscope and polariscope, will be much cheaper. If you buy
a cheap Chinese refractometer, you will need to make sure it is
accurate by testing it with quartz and some other stones with
constant, invariant RIs.
Someone suggested learning the Hodgkinson method and IDing with no
instruments at all. This is a great approach, but the info on it is
hard to find and one really has to understand what’s happening beyond
the “go-no go” approach of using a few simple instruments to do
Another simple approach with loose stones would be to learn how to
use a small digital scale with a tare function (about a $20 item)
along with a small condiment cup of water and a piece of string to
determine specific gravity. Quick and painless and a lot cheaper than
a refractometer. Weigh the gem suspended on the string in the water,
then tare the water in the cup at zero and weigh the stone by
allowing it to rest on the bottom of the cup in the water.
Then make the SG calculation. A lot easier than learning to use a
refractometer well, and no stones over the limit. Refractometers are
great when you don’t know much and want to use a look-up table and
when there is no synthetic with the same RI or material with a near
or overlapping RI. Also when the stone is mounted so that an SG
reading instead is impossible.
Another problem with “buy a refractometer” is that the OP’s needs
may change, and some questions would eventually require some
gemological knowledge. A good starting point for this would be
Antoinette Matlins’ book, Gem Identifaction Made Easy, now in its
third or fourth edition. Lots of used copies around.
If you really want to understand gemology, Wm. Hanneman’s Guide to
Affordable Gemology will teach the thinking person all they need to
know better than any of the more traditional gemology texts.
The place to ask gemology questions of the experts is at the
Gemology On Line forums. Dr. Hanneman and other experts hang around
there. Sorry to be so long winded, but the specific answers to
gemological questions are often a little more involved than “buy a
refractomter.” That’s like telling somebody who wants to make jewelry
to “buy a torch.” Uh, well, yes, maybe, but…