The other rare colour change "Alexandrite" from Sri Lanka is actually a blue-green sapphire which changes to purple colour under artificial light.

However, statements here will concentrate on the original chrysoberyl category.

Alexandrite has a positive electrical charge which stays on for hours after rubbing and radiates sensitivity. Used in holistic healing, it is useful that it bypasses the actual ailment by going directly to the root of the problem. Recommended as a gemstone for Ketu, the astral healing of Alexandrite acts directly on the head, neck muscles and spinal areas, enabling a re-balancing of the different locations of energies. It also helps to improve memory and eyesight. All importantly, it transmits inner peace by creating magnanimity of the heart.

The original source in Russia's Ural Mountains has long since closed after producing for only a few decades and only a few gemstones can be found on the market today and a certificate of Russian origin is still valued by the gem trade. Some Alexandrite can be found in Sri Lanka, Zimbabwe, Tanzania and Brazil but very few give a dramatic colour change. The largest rough ever found was in Sri Lanka weighing 1876 carats, but one can view the largest faceted 66 carat Alexandrite at the Smithsonian Institute, Washington.

Besides the four "C’s" in carat, cut, clarity, when evaluating Alexandrite, pay the most attention to the colour change. The more dramatic and complete the shift from red to green, without the "bleeding" through of brown from one colour to the next, the more rare and valuable the gemstone. It is recommended you take the gemstone to a shady area, never in direct sunlight to look at its natural colour. For the colour change, take the gemstone into a darkened room where only an incandescent light is the only source of light. The more intense the colour change, the better. Because of the rarity of this gemstone, larger sizes command very, very hefty price tags > US$30,000/- per piece of gemstone.

Like all garnets and spinels, there are no known treatments on the Alexandrite.

Alexandrite should not be cleaned in an ultrasonic machine or a steam cleaner.  Ionic cleaners and/or warm, soapy water and a soft brush is the best care. Care has to be taken in that it is sensitive to knocks and it is not resistant to alkalis.

Mohs’ scale : 8.5

Beryllium aluminium oxide

Vaidurya or Cat’s Eye is the name given to the variety of chrysoberyl which beautifully displays the phenomenon of chatoyancy.  The fine parallel lines of inclusions in the crystal is aligned at a 60º angle with special oil to catch the best effects of its asterism, and is then fashioned into cabochons to emphasize the "eye effect".

Similarly, there are members of the quartz family that will also give a fine asterism in the final polish.

The variegated colour is assigned to Ketu, and it is believed that the best gemstone assigned to promote Ketu’s blessings in attaining moksha, are gems that have the mysterious colour change.