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Article: Chrysoberyl


Chrysoberyl Properties and Varieties

  • Chemical formula: 4.BA.05
  • Mohs hardness: 8.5
  • Luster: Vitreous
  • Streak color: White
  • Specific gravity: 3.5-3.84
  • Three main varieties: ordinary yellow-to-green chrysoberyl, cats eye or cymophane, and alexandrite
  • Chrysolite was a historical name for yellow-green chrysoberyl
  • Alexandrite is strongly pleochroic and changes color in different lighting conditions
  • Cats eye chrysoberyl exhibits chatoyancy or opalescence
  • Cymophane forms light-green specimens with a silky band of light

Chrysoberyl Crystals and Occurrence

  • Crystals can exhibit cyclic twins called trillings
  • Trillings have a hexagonal appearance
  • V-shaped twins result when only two twin orientations are present
  • Ordinary chrysoberyl is yellowish-green and transparent to translucent
  • Used as a gemstone when it exhibits good pale green to yellow color and transparency
  • Forms as a result of pegmatitic processes
  • Can be found in mica schists and in contact with metamorphic deposits of dolomitic marble
  • Weathered out of rocks and deposited in river sands and gravels in alluvial deposits
  • Often recovered from placers in Brazil and Sri Lanka
  • Crystals can grow quickly in pegmatites due to high water content in the magma

Alexandrite Color Change and Identification

  • Alexandrite displays a color change dependent on ambient lighting
  • Color change is caused by small scale replacement of aluminium by chromium ions
  • Appears greenish in daylight and reddish in incandescent light
  • Color change is independent of pleochroism
  • Stones with dramatic color change and strong colors are rare and sought-after
  • Natural alexandrite contains inclusions that appear natural
  • Czochralski or pulled alexandrite is identifiable by its clean appearance and curved striations visible under magnification
  • Hydrothermal lab-grown alexandrite has identical physical and chemical properties to real alexandrite
  • Some gemstones falsely described as lab-grown synthetic alexandrite are actually corundum laced with trace elements or color-change spinel
  • These falsely described gemstones should be referred to as simulated alexandrite rather than synthetic

Cymophane (Cat's Eye Chrysoberyl) and Cats Eye Effect

  • Cymophane is translucent yellowish chatoyant chrysoberyl
  • It gets its name from the Greek words meaning wave and appearance, referring to the haziness that distorts its surface
  • Cymophane exhibits a cat eye effect caused by microscopic tubelike cavities or needle-like inclusions of rutile
  • The chatoyant effect is best seen in gemstones cut in cabochon form perpendicular to the c-axis
  • Only chrysoberyl can be referred to as a cat's eye without any other designation
  • Gems lacking the silky inclusions required for the cat's eye effect are usually faceted
  • An alexandrite cat's eye is a chrysoberyl cat's eye that changes color
  • The best cat's eyes are described as having a milk and honey color
  • The cat's eye effect refers to the sharp milky ray of white light crossing the cabochon as a center line

Popularity and Value of Cats Eye

  • Cats eye became more popular in the late 19th century when the Duke of Connaught gave a ring with a cat's eye as an engagement token
  • Before that, cat's eye had mainly been present in gem and mineral collections
  • The increased demand for cat's eye increased its value and led to intensified searches for it in Sri Lanka
  • The honey color is considered top-grade by many gemologists, but lemon yellow colors are also popular
  • Cats eye material is found as a small percentage of the overall chrysoberyl production

Chrysoberyl Data Sources

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