that does not show a color change between daylight and artificial light (the
chrysoberyl which shows a colour change is called alexandrite). The ideal
colors of chrysoberyl are green and yellowish-green. In addition, due to
strong dichroism, one may see an attractive bi-coloured chrysoberyl
occasionally. Hardness is 8.5 on the Moh's scale. The high refractive index
of the stone makes it very lively when properly cut and polished.
The mineral or gemstone chrysoberyl, not to be confused with beryl, is an
aluminate of beryllium. Chrysoberyl is transparent to translucent and
sometimes chatoyant. An interesting feature of uncut crystals of chrysoberyl
are the cyclic twins called trillings. These twinned crystals have a
hexagonal appearance, but are the result of a triplet of twins with each
"twin" taking up 120 degrees of the cyclic trilling. The word chrysoberyl is
derived from the Greek chrysos, "golden," and beryllos, of uncertain
etymology.
Varieties - Alexandrite, Color change green to red, Chatoyant, Various
shades of green and yellow; brownish, reddish.
Sources - Sri Lanka, India, Brazil, Zimbabwe, Myanmar.
History - The name chrysoberyl comes from the Greek, chrysoberyl meaning
golden beryl. But chrysoberyl is more hard and therefore different from the
classical gem beryl, which is much softer. Chrysolite is another name given
to the light greenish yellow variety of chrysoberyl that was in fashion
during the nineteenth century. A magnificent 47 carats chrysoberyl is listed
in the catalog of the British Museum of Natural History.
Cut and uses - Chrysoberyl and Alexandrite is usually faceted. Chrysoberyl
cat's eyes must be cut en cabochon to display a chatoyant effect.
Translucent yellowish chatoyant chrysoberyl is called cymophane or cat's
eye. Cymophane has its derivation also from the Greek words meaning wave and
appearance, in reference to the chatoyancy sometimes exhibited. In this
variety, microscopic tubelike cavities or needlelike inclusions of rutile
occur in an orientation parallel to the c-axis producing a chatoyant effect
visible as a single ray of light passing across the crystal. This effect is
best seen in gemstones cut in cabochon form perpendicular to the c-axis.
Although other minerals such as tourmaline, scapolite, corundum, spinel and
quartz can form "cat's eye" stones similar in appearance to cymophane, the
jewelry industry designates these stones as "quartz cat's eyes", or "ruby
cat's eyes" and only chrysoberyl can be referred to as "cat's eye" with no
other designation.
There are main three chrysoberyl varieties. The first type is simply faceted
transparent Chrysoberyl that is usually found in yellowish green to green,
yellow and shades of brown. It is a fine gemstone, but is over-shadowed by
its two cousins. The second variety is the "cat's eye", also known as
cymophane. The effect is caused by microscopic needle-like inclusions that
reflect light into a single dynamic sliver of light running along the center
of the crystal thus making it look like a living cat's eye! The third and
perhaps most interesting is Alexandrite. This rare and valuable gemstone has
the unique property of changing color depending on the type of light that
hits it. In sunlight, it appears almost emerald green, while in artificial
incandescent light it appears a violet-red. Some sapphires show similar
ability, and synthetic sapphires are now on the market being sold as
"Alexandrites" but at substantially lower prices than natural Alexandrite.
Until recently the main producers of fine quality chrysoberyl were Brazil
and Sri Lanka, but now much more productive mines are in Madagascar and
Tanzania. Because Sri Lanka Chrysoberyl has the finest quality and high
value most of the Madagascar rough is sold as genuine Sri Lanka stones.
When choosing a Chrysoberyl gemstone, due to the pastel color of
chrysoberyl, special attention should be given to the cut and overall
proportions of the stone. When choosing a cat's eye gemstone priority should
be given to the ray or eye of the stone.
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