Alexandrite & Cats’ Eye
Chrysoberyl is a phenomenal gem.  Alexandrite and cats’ eye are varieties of chrysoberyl and exhibit two of the most startling light phenomena present in gemstones, namely chatoyancy in Cats’ Eye and change-of-color in Alexandrite.  Chrysoberyl also occurs in a lovely transparent greenish- to brownish-yellow color, called Chrysolite, which was very popular in the Georgian, Victorian and Edwardian eras as a substitute for diamonds.  Chrysoberyl, in all its forms at 8½ on the Mohs’ scale of hardness, is harder than any other gem except for diamond and sapphire.  This hardness allows chrysoberyl to be set in all forms of jewelry, including rings.
Alexandrite was first discovered in the Ural mountain emerald mines located north-east of Sverdlovsk (Ekaterinburg), Russia, in 1830 on the anniversary of the birth of then Czar Alexander II.  It was adopted as that country’s national gem for nearly 100 years.  Alexandrite has the ability to change color depending upon what type of light is used to view it.  In daylight or fluorescent light, alexandrite ideally appears bluish- or yellowish-green and changes to violetish-red under incandescent or candle light.  The clarity and purity of hue greatly affect the value, and the more pure the hue the higher the quality of the alexandrite.  This phenomenon is caused by selective absorption, where the material absorbs all colors but one in a particular type of light source.  When the type of light is changed, the material absorbs all but a different, usually contrasting, color.  Alexandrite is now mined in Brazil, Sri Lanka (Ceylon), Zimbabwe, Burma (Myanmar), Madagascar and Tanzania.  The largest known gem-quality faceted alexandrite is a mixed cushion shape cut weighing 65.70 carats and resides in the Smithsonian Institution in Washington, D.C.  It was mined in Sri Lanka and purchased using funds donated by Lelander T. Chamberlain specifically for the National Gem Collection.  The gem is lightly included and has a moderate color change from yellowish-green to reddish-violet.
Cats’ Eye chrysoberyl is also called cymophane, Greek for wavering light.  Very fine aligned parallel inclusions when oriented parallel to the surface of a cabochon cut stone exhibit a streak of light crossing the stone.  This is known as the phenomena of chatoyancy and stones possessing this feature are considered chatoyant.  It is important for the cutter to orient the phenomena appropriately so that the “eye” is well centered and preferably in the direction of the long axis of the stone, unless the stone is cut in a round shape.  Although other gemstones exhibit the cats’ eye effect, only chrysoberyl has an “eye” that opens and closes when exposed to two light sources at once.  The finest cats’ eyes also exhibit what is called the “milk and honey” effect, where when exposed to a single light source, one side of the “eye” is transparent and the other side is translucent.  The name cats’ eye used alone as a designation refers to the chrysoberyl variety, whereas other species with cats’ eyes are named as such prefixed with the cats’ eye designation.  Cats’ eyes occur primarily in brownish- to greenish-yellow hues.  The best cats’ eyes tend towards the browner tones.  Very rarely an alexandrite will also exhibit the cats’ eye effect, resulting in two different types of light phenomena in the same gemstone.  Major deposits of cats’ eye chrysoberyl occur in Sri Lanka, Brazil, China, India and Zimbabwe.
Chrysoberyl Properties
Species:  Chrysoberyl (Criss-o-bar-rel)
Varieties:  Alexandrite, Cats’ Eye, Cats’ Eye Alexandrite, Cymophane, Chrysolite
Chemical Composition:  BeAl2O4
Color:  Alexandrite - Green, Red, Violet; Cats’ Eye - Brownish-Yellow, Greenish-Yellow, Green, Brown, Colorless, Red, Violet
Hardness:  8½
Crystal System:  Orthorhombic
Refractive Index:  1.746 - 1.763
Specific Gravity:  3.70 - 3.78

Images by Paul Wild.
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