Alexandrite Chrysoberyl

Alexandrite is composed of beryllium aluminium oxide. It has two types of variants. One shows color changing effect while other exhibits cat's eye effect.

Properties

Chemical Composition: 

Beryllium Aluminium Oxide - BeAl₂O₄

Classification / Type: 

-

Colors / Varieties: 

• Alexandrite (color change in lamplight and daylight)
• Alexandrite Cat's Eye

Crystal System / Forms: 

Orthorhombic System

Hardness: 

8.5

Specific Gravity: 

3.71 - 3.75

Cleavage / Fracture: 

Imperfect / Conchoidal fractur

Optic Character: 

Anisotropic, D.R.; Biaxial positive.

Lustre: 

Vitreous

Refractive Index / Birefringence: 

1.746 - 1.755 / 0.009; Range: 1.740 - 1.760

Pleochroism: 

Alexandrite exhibits strong trichroism (red, orange, green) while other varieties exhibit weak to moderate effect.

Dispersion: 

0.015

Magnification: 

Phase and fingerprints, crystalline inclusions, growth zoning - trilling boundaries.

U.V. Fluorescence: 

Inert

Spectrum: 

• Broad absorption in the yellow-green region at 575nm and in the violet, and narrow lines in the red at 680nm and 678nm, weak lines at 665nm, 655nm, 649nm and in blue portion at around 470nm.

Cause of Color: 

Chromium

Treatment (Enhancement): 

Lighter yellow, included cat's eye irradiated to a more transparent deep brown color. Most of these were found to be radioactive.

Specific Tests & Remarks: 

Alexandrite Color

• Green (greenish blue, bluish green, brownish green) in daylight and red (purplish red, brownish red, grayish red) in incandescent light.
• This is due to small scale replacement of Aluminium Oxide by Chromic Oxide.
• The transmission of the red part of the spectrum is more or less balanced and any change in the nature of the incident light will either emphasize the red or the green-blue.
• Daylight is rich in blue wavelength while incandescent light (other than mercury vapor fluorescent lamps) is richer in red wavelengths and hence the stone exhibits the green to red color change.

Synthesis: 

Czochralski and Flux fusion method.

• Synthetic Alexandrite
• Synthetic Alexandrite Cat's Eye

Simulants (with key separation tests): 

• Natural / Synthetic Color Changing Sapphire (optic figure, R.I., S.G., inclusions), Color Changing Garnet (optic character, spectrum), Tourmaline (optic figure, R.I., S.G.), Andalusite (R.I., S.G., pleochroism), Epidote (birefringence, spectrum), Natural / Synthetic Color Changing Spinel (optic character, pleochroism)

Geological Occurrence: 

In pegmatite, schist, marbles and alluvial deposits.

Sources: 

Russia, Tanzania, Sri Lanka, Brazil, Zimbabwe, India (Orissa, Tamil Nadu, Andhra Pradesh), Myanmar (Burma), Madagascar.

Cuts & Uses: 

Facetted cuts, cabochon, generally mixed cuts, also as beads, carvings, etc.