One of the biggest challenges for a new gemologist is separating diamonds from their look alikes. The keys to identifying these gems are specific gravity and birefringence.

The following stones are arranged by specific gravity. Note that there are only three pairs of stones that share the same specific gravity, (the ones in red.) In each case, one is doubly refractive and the other is not. This makes the separations simple.

Material SG high SG low Birefringence
GGG GARNET 7.09 6.95 None
CUBIC ZIRCONIA 6.00 5.34 none
RUTILE 5.60 4.20 .287
ZIRCON 4.80 3.90 .000 – .059
YAG GARNET 4.60 4.50 None
SPHENE 3.55 3.45 .100 – .192
DIAMOND 3.53 3.51 None
MOISANITE* 3.20 3.17 .043

It appears that zircon, which is occasionally isometric, could be confused with YAG. However, anytime zircon becomes singly refractive, the specific gravity drops below that of YAG.

* Moisanite has the unique property of being theromoluminescent. By gradually heating it, it will change color. Around 150 degrees Fahrenheit it starts turning light brown and as the heat increases it becomes bright yellow or green. Of course, you need to do this very gradually, as a rapid temperature change could damage some of the other stones.


Compliments ofThe International Gem Society

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