Synthetic forsterite

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From The Gemology Project
Synthetic forsterite
Chemical composition Magnesium silicate

Mg2SiO4

Crystal system Orthorhombic
Hardness 7
Optic nature Biaxial +
Refractive index 1.634 - 1.670
Birefringence 0.033 - 0.038
Specific gravity 3.22
Pleochroism Strongly dichroic
Fluorescence

LW: chalky orangy yellow
SW: weak greenish yellow

Synthetic forsterite
Photo courtesy of Chaman Golecha
Gem Testing Laboratory, Jaipur


Although forsterite does occur in nature, it is not suitable to be cut into gemstones. As from 1999 large size synthetic forsterites have been created in Russia by the Czochralski pulling technique.
Due to physical and optical properties that lie close to tanzanite, this synthetic is used mainly to imitate tanzanite.

Contents

Chemical composition

Mg2SiO4, magnesium sillicate.
Forsterite is one of the endmembers of the isomorphous group to which peridot (olivine) belongs (fayalite-forsterite series).

Diagnostics

One of the easiest and most important techniques in separation synthetic forsterite from tanzanite is by observing the "doubling of back facet edges" (double refraction) due to the high birefringence of synthetic forsterite. This feature is easily visible on examination in various directions using a loupe.
Tanzanite with a birefringence of 0.008 to 0.013 will not show such a strong doubling effect.

Hanneman Tanzanite Filter

Another quick method to seperate synthetic forsterite from tanzanite is with the aid of a Hanneman Tanzanite Filter. This is a filter that combines the Hanneman Aquamarine filter with a London dichroscope.
Synthetic forsterite will show up green through the filter part, while tanzanite will show a pinkish-orange color.

Refractometer

The refractive indices of synthetic forsterite are lower than that of tanzanite.
1.634-1.670 (tanzanite: 1.685-1.707).
Forsterite's birefringence is much higher than that of tanzanite: 0.033 - 0.038 for forsterite, 0.008 - 0.013 for tanzanite.

The optic character of pure forsterite is biaxial with a positive optic sign (the same as tanzanite). One will sometimes read forsterite to be biaxial -, however if olivine consists of 100 to 85% forsterite (0 to 15% fayalite) it is biaxial +.
In practise synthetic forsterite will be biaxial with a positive optic sign.

Fluorescence

Synthetic forsterite will show a weak greenish-yellow under SW-UV and a chalky orangy-yellow under LW-UV. Tanzanite is inert to both.

Update

Natural colorless forsterite.
Image courtesy of Scott Davies


Natural colorless forsterite has been found in Mogok, Burma (Myanmar) as of early 2008. Originally thought to be a member of the humite group, testing at the GIT and the GIA Laboratory in Bangkok confirmed that the stones were forsterite.

References



Next: Synthetic Moisanite

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