Chrysoberyl

Chrysoberyl occurs in a variety of colours, from green, greenish yellow, and yellow to brown. It is a hard, durable stone, particularly suitable for use in jewellery. When cut well, gems are brilliant but lack fire. Two varieties, alexandrite and cat's-eye, have unique qualities of their own. The very rare and valuable alexandrite changes from green in daylight to red, mauve, or brown under incandescent light, such as a light bulb. Synthetic chrysoberyl, synthetic corundum, and synthetic spinel have all been produced to imitate alexandrite’s colour change.

Cat's-eye, when cut in cabochon, has a near-white line across a yellowish grey stone, due to canal or feather-like fluid inclusions, or needle-like inclusions of rutile. The most highly prized cat's-eye color is a light golden brown, often with a shadow that gives a light and dark, "milk and honey" effect. Pale yellow chrysoberyl, popular in 18th and 19th century Portuguese jewellery, was also knonwn as chrysolite.

Although mainly worked out, the best chrysoberyl, including alexandrite, has been discovered in mica schists in the Urals (Russia). The largest faceted chrysoberyl from Russia weighs 66 carats. Large waterworn pebbles of various colors are found in the gem gravels of Sri Lanka. Chrysoberyl also occurs in Burma, Brazil, Zimbabwe, Tanzania and Madagascar. Cat's eye is found in Sri Lanka, Brazil and China.

The name is from the Greek "chrysos", meaning golden and "beryllos" which refers to the beryllium content. Known for centuries in Asia, it was highly valued for the protection it afforded from the "evil eye".

Chrysoberyl Properties

Chemical Composition: 
Beryllium Aluminium Oxide - BeAl2O4
Colors / Varieties: 
  • Chrysoberyl: Transparent green, yellow, brown
  • Chrysoberyl Cat's Eye: Yellow / green / brown
  • Alexandrite (color change in lamplight and daylight)
  • Alexandrite Cat's Eye
Crystal System / Forms: 
Orthorhombic System / Found as prismatic crystals usually flattened parallel to the macro - pinacoidal faces. Also as interpenetrant and cyclic twins (Trilling - forms produced by triple twinning as is seen in the pseudo-hexagonal cyclic twin).
Hardness: 
8.5
Specific Gravity: 
3.71 - 3.75
Cleavage / Fracture: 
Imperfect / Conchoidal fracture
Optic Character: 
Anisotropic, D.R.; Biaxial positive.
Lustre: 
Vitreous
Refractive Index / Birefringence: 
1.746 - 1.755 / 0.009; Range: 1.740 - 1.760
Pleochroism: 
Weak trichroism
Dispersion: 
0.015
Magnification: 
Phase and fingerprints, crystalline inclusions, growth zoning - trilling boundaries.
U.V. Fluorescence: 
Inert
Spectrum: 
  • Yellow / Brown / Green Varieties: Strong band at 444nm.
Cause of Color: 
Yellow, green, brown: Iron
Treatment (Enhancement): 
Lighter yellow, included cat's eye irradiated to a more transparent deep brown color. Most of these were found to be radioactive.
Synthesis: 
Czochralski and Flux fusion method.
  • Synthetic Chrysoberyl Cat's Eye
Simulants (with separation tests): 
  • Quartz (R.I., S.G., lustre), Apatite (R.I., S.G., lustre, inclusions), Sillimanite (R.I., S.G., spectrum), Glass (inclusions)
Geological Occurrence: 
In pegmatite, schist, marbles and alluvial deposits.
Sources: 
Sri Lanka, India, Brazil, Madagascar.
Cuts & Uses: 
Facetted cuts, cabochon, generally mixed cuts, also as beads, carvings, etc.