Difference between revisions of "Synthetic forsterite"

From The Gemology Project
Jump to: navigation, search
(Diagnostics)
Line 26: Line 26:
  
 
The optic character of pure forsterite is biaxial with a positive optic sign (the same as tanzanite). One will sometimes read fosterite to be biaxial -, however if olivine consists of 100 to 85% forsterite it is biaxial +.
 
The optic character of pure forsterite is biaxial with a positive optic sign (the same as tanzanite). One will sometimes read fosterite to be biaxial -, however if olivine consists of 100 to 85% forsterite it is biaxial +.
 +
 +
===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.

Revision as of 03:37, 18 August 2007

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.

Chemical composition

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

Diagnostics

Hanneman Tanzanite Filter

Probably the quickest 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.
1.635-1.670 (tanzanite: 1.685-1.707).
Forsterite's birfringence is also much higher than that of tanzanite: 0.035 for forsterite, 0.008 for tanzanite.

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

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.