Alexandrite


Alexandrite is chrysoberyl BeAl2O4 with Cr3+ as an impurity. Alexandrite is amazing gemstone for jewelry, because faceted gems undergo a dramatic color change in different types of light: in fluorescent light blue dominates, while red is predominant when viewed in light from a tungsten lamp. The impurity that produces the color in alexandrite is Cr3+, this is the so called ‘alexandrite effect’. The most desirable color change is rich blue–green to a deep raspberry red, and the shade of the color can vary. The gem was named after Czar Alexander II of Russia because it was discovered in 1830, the year of his coming of age. The best natural source of alexandrite is the Ural Mountains in Russia. In 1987, a new find of alexandrite was made in Brazil. Later, alexandrite discoveries were made in Sri Lanka, Madagascar, Myanmar, and Zimbabwe. However, none of these sites produces as rich and vivid a color change as the original Russian source.

Lab-grown crystals usually have higher perfection than is normally seen in natural stones. 

PROPERTIES

Chemistry: BeAl2O4 + Fe, Ti
Crystal structure: orthorhombic
Hardness: 8.5
Refractive index: 1.746 - 1.756
Density: 3.75
Cleavage: distinct to poor, 1 direction

HISTORY

Synthetic alexandrite is a recent achievement: synthetic alexandrite gemstones have been produced since 1973 and patented by Creative Crystals Inc. of San Ramon, California. True synthetic alexandrite has a color change and characteristics analogous to those of natural stones from Siberia and do not just imitate them in color like corundum with vanadium or chlorine inclusions.

SYNTHESIS


Synthetic alexandrite is mostly grown by Chokhralski method (pulled alexandrite) and by floating-zone method.
The Chokhralski process, or crystal pulling technique, is a method of crystal growth used to obtain crystals (especially rod-shaped single crystals), metals and salts. The process starts with melting the materials of appropriate composition in a crucible. To make alexandrite crystals by Chokhralski method, beryllium and aluminum oxides were melted together in a crucible with addition of vanadium, chromium and ferric oxides. A seed crystal mounted on a rod is immersed into the melt where it starts to rotate. The crystal's rod is then pulled upwards slowly and rotated at the same time. By precisely controlling the temperature gradients, rate of pulling and speed of rotation, it is possible to extract a large, single crystal from the melt.

Synthetic alexandrite can also be created by the floating-zone method. This technique is crucible-free; hence the possibility of contamination from crucibles is eliminated: the purity of the resulting crystal will be at least as good as that of the initial polycrystalline starting material. The method is also used to synthetically grow a variety of crystals: YAG (Yttrium Aluminum Garnet), Ruby and Chrysoberyl. Floating-zone method is similar to the Chokhralski pulling method, when the crystal is pulled vertically, but, instead, here it is pulled horizontally, using a tungsten dish and slow-melting technique, where the dish is pulled over a period of time under a heating element. The period of time depends on the size of the dish and the materials needed to be crystallized. In the case of alexandrite, the speed of the process is about 2 mm/hour. An average crystal being 18 cm long would take about 15 days to pull. This method was developed by a Russian-Armenian scientist, Professor Bagdasarov, in the 1970’s and was widely used in production of YAG before finding its way into the jewelry scene.

Typical inclusions in synthetic alexandrite consist of a layer of dust-like particles parallel to the seed plate, and strong banding or growth lines. Unlike their natural counterparts, synthetic alexandrite stones sometimes have a high SWUV transparency factor. Synthetic alexandrite can also have a swirled internal structure which is characteristic of the floating zone method of synthesis. This alexandrite can have tadpole inclusions (with long tails) and spherical bubbles. The color change in such stones is reasonable, but not suspiciously good.

Synthetic alexandrite is used

  • As a gemstone
  • In laser applications