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Benitoite

benitoite
Benitoite rough

The blue crystals of benitoite were only discovered in 1906, by a mineral prospector who mistook them for sapphires. Crystals are shaped like flattened triangles, and have a strong dispersion similar to diamond, but this is masked by the colour. Dichroism is strong: the stone appears blue or colourless when viewed from different angles. Colourless crystals occur, but are rarely faceted.

Crystals occur in veins in blue schists. The sole source is in San Benito County, California (USA), after which the stone is named.

 

Cause of Color : Iron: titanium charge transfer.
Chemical Composition :

Silicate of Barium and Titanium – BaTiSi3O9


Crystal System / Forms :

Hexagonal System / Flat triangular crystals.


Cuts & Uses :

Beads, facetted and other cuts. Generally the table facet is cut parallel to the vertical crystal axis.


Dispersion : 0.046 (body color masks the dispersion)
Hardness : 6.5
Lustre : Vitreous
Magnification :

Crystalline inclusions, doubling, color zoning.


Optic Character :

Anisotropic, D.R.; Uniaxial Positive


Pleochroism : Strongly dichroic: Blue and colorless.
Refractive Index / Birefringence :

1.755 – 1.804 / 0.047


Simulants (with separation tests) :

Natural / Synthetic Sapphire (R.I., S.G., doubling, dispersion), Tanzanite (optic figure, pleochroism, doubling, dispersion), Irradiated Blue Beryl (pleochroism, R.I., S.G., doubling, dispersion)


Sources :

U.S.A. (San Benito County in California)


Specific Tests :

High birefringence and dispersion.


Spectrum :

Not characteristic.


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