Benitoite

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.

Properties

Chemical Composition: Silicate of Barium and Titanium - BaTiSi₃O₉

Classification / Type: -

Colors / Varieties: Blue, violet blue, colorless, pink.

Crystal System / Forms: Hexagonal System / Flat triangular crystals.

Hardness: 6.5

Specific Gravity: 3.64 - 3.68

Cleavage / Fracture: Poor, indistinct / Conchoidal fracture

Optic Character: Anisotropic, D.R.; Uniaxial Positive

Lustre: Vitreous

Refractive Index / Birefringence: 1.755 - 1.804 / 0.047

Pleochroism: Strongly dichroic: Blue and colorless.

Dispersion: 0.046 (body color masks the dispersion)

Magnification: Crystalline inclusions, doubling, color zoning.

U.V. Fluorescence: Strong chalky blue in shortwave and dull red to inert in longwave

Spectrum: Not characteristic

Cause of Color: Iron: titanium charge transfer

Treatment (Enhancement): Colorless or blue stones heated to give a pinkish orange color.

Specific Tests & Remarks: High birefringence and dispersion.

Synthesis: -

Simulants (with key 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)

Geological Occurrence: 

Occurs along with compact natrolite associated with neptunite.

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

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