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