Jewellers' Network

Tanzanite Imitations Cultured Pearls

BY Craig Thomas (FGA)(GD)(AJP) Head of Laboratory A.G.L.

Ever since man has valued gems there have been those that try to deceive potential buyers with imitation gems, and tanzanite is no exception. So let’s take a look at some of the potential candidates as tanzanite imitations.

Natural Tanzanite

Crystal system: Orthorhombic
Optically: Anisotropic
Mohs Hardness: 6.5
Specific Gravity: 3.10-3.38
Refractive Index: 1.69-1.70
Birefringence: 0.006-0.018


Glass/Paste

There have been quite a few of these around lately and they often come accompanied by a laboratory certificate. We can’t be sure whether the certificate is fake or if the stone has been manufactured to try and fit the description of a legitimate laboratory certificate. The ones we have seen had inaccurate weights and dimensions so this is a good place to start when trying to verify a stone. The glass is isotropic and lacks the dichroic colours of tanzanite. The refractive index (+-1.65) is usually lower than tanzanite with a single shadow edge.

Crystal system: None (amorphous)
Optically: Isotropic
Mohs Hardness: +-6
Specific Gravity: Usually +-3.0-4.0
Refractive Index: Usually +- 1.6-1.7


Cubic Zirconia

CZ is not as convincing as the glass imitation. The high dispersion of CZ causes flashes of rainbow colours that make the stone appear slightly different from tanzanite. The specific gravity of CZ is almost twice that of tanzanite so the size to weight ratio would indicate that something is wrong and with it’s refractive index over the limit of the refractometer, CZ is an easy separation from tanzanite.

Crystal system: Cubic
Optically: Isotropic
Mohs Hardness: 8-8.5
Specific Gravity: 5.60-6.00
Refractive Index: 2.15-2.18 (OTL)


Synthetic Forsterite

Synthetic forsterite is a very convincing tanzanite imitation. Being anisotropic it has a similar dichroism to tanzanite although there is a filter that can differentiate it from tanzanite based on a difference in its selective absorption of white light. Synthetic forsterite has a birefringance that is more than double that of tanzanite resulting in obviously wide “tram line” doubling of the pavilion facets. This large birefringance together with its refractive index of 1.63-1.67 distinguish it from tanzanite.

Crystal system: Orthorhombic
Optically: Anisotropic
Mohs Hardness: 7
Specific Gravity: 3.26
Refractive Index: 1.63-1.67
Birefringence: 0.035


Iolite

This natural stone is sometimes confused with tanzanite and when it is cut right it can be a convincing tanzanite imitation. If you are able to observe iolite from different angles you will notice an obviously brownish axis in the stone. This difference in dichroism coupled with its Refractive index of 1.54-1.57 is enough to separate iolite from tanzanite.

Crystal system: Orthorhombic
Optically: Anisotropic
Mohs Hardness: 7-7.5
Specific Gravity: 2.58-2.66
Refractive Index: 1.53-1.57
Birefringence: 0.008-0.012


Conclusion

So when one is considering buying a tanzanite or any stone for that matter, don’t just trust the certificate without checking the stone first to ensure that it is a tanzanite and that the details accurately match those on the certificate. When there is any doubt it is safer to send it in to a laboratory for verification before parting with any hard earned money.