Pulled Tanzanite (Synthetic forsterite)
Although forsterite does occur in nature, it is generally not suitable for cutting into gemstones.
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Pulled Tanzanite: Synthetic Forsterite
Since 1999, large synthetic forsterites have been produced in Russia using the Czochralski pulling technique. Due to their physical and optical properties, which are quite similar to tanzanite, these synthetics are often used to imitate tanzanite in the gemstone market.
Chemical Composition: Mg2SiO4, Magnesium Silicate
Forsterite is one of the end members of the isomorphous series that includes peridot (olivine), specifically the fayalite-forsterite series.
Identifying Synthetic Forsterite vs. Tanzanite
Double Refraction
One of the simplest and most effective methods for distinguishing synthetic forsterite from tanzanite is by observing the “doubling of back facet edges,” a phenomenon caused by the high birefringence of synthetic forsterite. This effect, which is due to double refraction, is easily visible when examining the gemstone from different angles using a loupe. In contrast, tanzanite, which has a birefringence of 0.008 to 0.013, does not exhibit this strong doubling effect.
Hanneman Tanzanite Filter
Another effective technique for distinguishing synthetic forsterite from tanzanite is to use a Hanneman Tanzanite Filter. This filter combines the Hanneman Aquamarine filter with a London dichroscope. When viewed through the filter, synthetic forsterite will appear green, while tanzanite will display a pinkish-orange color.
Refractive Indices and Birefringence
Refractive Index Comparison
The refractive indices of synthetic forsterite are slightly lower than those of tanzanite. Specifically, the refractive index for synthetic forsterite ranges from 1.634 to 1.670, whereas for tanzanite it ranges from 1.685 to 1.707.
Birefringence Comparison
The birefringence of synthetic forsterite is also much higher: 0.033 to 0.038 compared to 0.008 to 0.013 for tanzanite.
Pulled Tanzanite (Synthetic Forsterite)
Optic Characteristics
The optic character of pure forsterite is biaxial with a positive optic sign, which is the same as tanzanite. Although there are references to forsterite being biaxial negative, forsterite with a composition of 100% to 85% pure forsterite (0% to 15% fayalite) will have a biaxial positive optic sign. In practice, synthetic forsterite will exhibit a biaxial positive character.
UV Fluorescence
Fluorescence Under UV Light
Under shortwave ultraviolet light (SW-UV), synthetic forsterite exhibits a weak greenish-yellow fluorescence, while under longwave ultraviolet light (LW-UV), it displays a chalky orange-yellow fluorescence. In contrast, tanzanite is inert under both shortwave and longwave UV light, meaning it does not fluoresce.
Occurrence of Natural Forsterite
Natural colorless forsterite has been discovered in Mogok, Burma (Myanmar) since early 2008. Initially thought to belong to the humite group, further testing at the GIT and the GIA Laboratory in Bangkok confirmed these stones to be forsterite.
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