Danburite

Generally colourless, danburite crystals can also be yellow or pink. They form wedge-shaped prisms, much like the ones from colourless topaz but distinguishable by cleavage (poor in danburite, perfect in topaz) and particular gravity (lower in danburite).
First found within the town of Danbury, Connecticut (USA). Gem-quality crystals happens to Burma, Mexico, Switzerland, Italy and Japan.

Properties

Chemical Composition: 

Borosilicate of calcium - CaB₂(SiO₄)₂

Classification / Type: 

-

Colors / Varieties: 

Colorless, white, pink, light to dark yellow, yellowish brown, brown.

Crystal System / Forms: 

Orthorhombic System / Prismatic and wedge shaped crystals are common.

Hardness: 

7

Specific Gravity: 

3.00

Cleavage / Fracture: 

None / Uneven Fracture.

Optic Character: 

Anisotropic, D.R.; Biaxial positive or negative (depends on wavelengths of light used)

Lustre: 

Vitreous.

Refractive Index / Birefringence: 

1.630 - 1.636 / 0.006

Pleochroism: 

Weak to none.

Dispersion: 

0.017

Magnification: 

Crystal inclusions, liquid tubes.

U.V. Fluorescence: 

Sky blue to bright blue-green in longwave. Thermo-luminescent: reddish tint when heated. (Phosphoresces)

Spectrum: 

Sometimes shows line at 580nm (didymium lines - rare earth spectrum).

Cause of Color: 

Color center.

Treatment (Enhancement): 

-

Specific Tests & Remarks: 

Slowly attacked by hydrochloric acid.

Synthesis: 

-

Simulants (with key separation tests): 

Topaz (S.G.), Andalusite (pleochroism), Beryl (optic figure, R.I., S.G., inclusions), Quartz (optic figure, R.I., S.G.), Tourmaline (optic figure, R.I., birefringence, inclusions)

Geological Occurrence: 

In carbonate veins in granite rocks.

Sources: 

Myanmar, Japan, Madagascar, Russia, Mexico.

Cuts & Uses: 

Facetted, cabochon, etc.