Separating diamond lookalikes from the real thing is one of the biggest challenges a new gemologist faces. Diamonds and their simulants can share some optical and physical properties. However,  testing two properties, specific gravity and birefringence, holds the key.
By Donald Clark, CSM IMG 1 minute read
YAG diamond simulants

12 round, brilliant-cut yttrium aluminum garnet (YAG) gemstones, 23.5 ctw. Photo courtesy of and Auctions at Showplace.

In the chart below, you’ll find common diamond lookalikes or simulants and real diamonds arranged by specific gravity. Please note, only three pairs of adjacent stones have overlapping specific gravity values, the ones in red. However, in each case, only one out of the pair is birefringent or doubly refractive. So, this makes the separations simple.

Diamond and Diamond Lookalikes Arranged by Specific Gravity

Material SG high SG low Birefringence
Gadolinium Gallium Garnet (GGG) 7.09 6.95 None
Cubic Zirconia 6.00 5.34 none
Rutile 5.60 4.20 0.287
Zircon 4.80 3.90 0.000-0.059
Yttrium Aluminum Garnet (YAG) 4.60 4.50 None
Sphene 3.55 3.45 0.100-0.192
Diamond 3.53 3.51 None
Moissanite 3.20 3.17 0.043
diamond lookalikes - rutile and CZ

Rutile (left) and cubic zirconia (right) have both been used as diamond lookalikes. Although their SG values overlap, rutile has birefringence while cubic zirconia doesn’t. Photo by yellowcloud. Licensed under CC By 2.0.

Notes on Zircon and Moissanite

Occasionally, zircon can have an isometric crystal structure. (Some zircons receive enough natural radiation over time that their crystal structure changes). Therefore, a zircon could be singly refractive. Presumably, this would mean you could confuse it with yttrium aluminum garnet (YAG). However, anytime zircon becomes singly refractive, its specific gravity drops below that of YAG.

Moissanite also exhibits thermoluminescence, a type of luminescence induced by heat, which diamonds lack. By gradually heating moissanite, you can make it change color. At around 150° F, it starts turning light brown. As the heat increases, it becomes bright yellow or green. Of course, you must do this very gradually. Since a rapid temperature change could damage some of the other stones, use this test judiciously.