One of the most spectacular pleochroic (multicolored) stones known to humans is Alexandrite. Named for Czar Alexander II of Russia because of its discovery in the Ural mountains on his birthday in 1830, alexandrite is the June birthstone. This chromium-doped chrysoberyl (Cr+3;BeAl204) gemstone is one of the few trichroic minerals, exhibiting various hues of red/green (C-axis), blue/purple (A-axis) and yellow/orange (B-axis) depending on the lighting conditions and orientation of the crystal. The exact coloration is also dependent upon the stone size and the amount of chromium that was included in the material. Typical natural and created stones have 0.10 to 0.25 atomic percent chromium, which results in a stunning array of stone coloration. Additionally, alexandrite has a chemically stable structure with excellent mechanical properties (Moh hardness of 8.5). Thus, a properly faceted stone will provide a spectacular appearance that will last forever.
The Moh hardness scale (1 easy through 10 very difficult) is a relative ranking of materials based upon their ability to be mechanically altered:
The material is produced by the Czochralski process, in which the solid material is slowly pulled from a liquid melt by initiation of growth on a high quality seed. The initial seed crystal is slowly raised from the high temperature liquid melt which is contained in an iridium crucible. The temperature of the melt is controlled by radio frequency heating. The surface of the melt is maintained around the point of solidification, i.e. 1,870°C for alexandrite. These synthetic crystals grow at a rate of approximately 1 mm/hour. The final diameter of the crystal boule is dictated by the size of the crucible, which can range from 3-4 inches in diameter and can reach 6-8 inches in length.