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
Reference | URL |
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Glossary | https://harryandcojewellery.com.au/blogs/glossary/chrysoberyl |
Wikipedia | http://en.wikipedia.org/wiki/Chrysoberyl |
Wikidata | https://www.wikidata.org/wiki/Q422179 |
Knowledge Graph | https://www.google.com/search?kgmid=/m/0d80_ |