Alexandrite

alexandrite
Alexandrite (color change in lamplight and daylight)

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.

It is named after the Russian tsar Alexander II (1818-1881), the very first crystals having been discovered in April 1834 in the emerald mines near the Tokovaya River in the Urals. The discovery was made on the day the future tsar came of age. Although alexandrite is a relatively young gemstone, it certainly has a noble history. Since it shows both red and green, the principal colours of old Imperial Russia, it inevitably became the national stone of tsarist Russia.

Beautiful alexandrite in top quality, however, is very rare indeed and hardly ever used in modern jewellery. In antique Russian jewellery you may come across it with a little luck, since Russian master jewellers loved this stone. Tiffany’s master gemmologist George Frederick Kunz (1856-1932) was also fascinated by alexandrite, and the jeweller’s firm produced some beautiful series of rings and platinum ensembles at the end of the 19th and beginning of the 20th century. Smaller alexandrites were occasionally also used in Victorian jewellery from England.

Source: www.gemstone.org

Alexandrite Pictures

Alexandrite earrings

Cause of Color :

Chromium

Chemical Composition :

Beryllium Aluminium Oxide – BeAl2O4

Crystal System / Forms :

Orthorhombic System

Cuts & Uses :

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

Dispersion : 0.015
Hardness : 8.5
Lustre : Vitreous
Magnification :

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

Optic Character :

Anisotropic, D.R.; Biaxial positive.

Pleochroism : Alexandrite exhibits strong trichroism (red, orange, green) while other varieties exhibit weak to moderate effect.
Refractive Index / Birefringence :

1.746 – 1.755 / 0.009; Range: 1.740 – 1.760

Simulants (with 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)
Sources :

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

Specific Tests :

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.
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.
Synthesis :

Czochralski and Flux fusion method.

  • Synthetic Alexandrite
  • Synthetic Alexandrite Cat’s Eye

Reader Interactions

Leave a Reply