Chrysoberyl
The mineral or gemstone chrysoberyl is an aluminate of
beryllium with the formula BeAl2O4.[4][5] The name chrysoberyl is derived from
the Greek words, meaning "a gold-white spar". Despite the similarity
of their names, chrysoberyl and beryl are two completely different gemstones,
although they both contain beryllium. Chrysoberyl is the third-hardest
frequently encountered natural gemstone and lies at 8.5 on the Mohs scale of
mineral hardness, between corundum (9) and topaz (8).
An interesting feature of its crystals are the cyclic twins
called trilling’s. These twinned crystals have a hexagonal appearance, but are
the result of a triplet of twins with each "twin" oriented at 120° to
its neighbors and taking up 120° of the cyclic trilling. If only two of the
three possible twin orientations are present, a "V"-shaped twin result.
Ordinary chrysoberyl is yellowish-green and transparent to
translucent. When the mineral exhibits good pale green to yellow color and is
transparent, then it is used as a gemstone. The three main varieties of
chrysoberyl are: ordinary yellow-to-green chrysoberyl, cat's eye or cymophane,
and alexandrite. Yellow-green chrysoberyl was referred to as
"chrysolite" during the Victorian and Edwardian eras, which caused confusion
since that name has also been used for the mineral olivine ("peridot"
as a gemstone); that name is no longer used in the gemological nomenclature.
Alexandrite, a strongly pleochroic (trichroic) gem, will
exhibit emerald green, red and orange-yellow colors depending on viewing
direction in partially polarized light. However, its most distinctive property
is that it also changes color in artificial (tungsten/halogen) light compared
to daylight. The color change from red to green is due to strong absorption of
light in a narrow yellow portion of the spectrum, while allowing large bands of
more blue-green and red wavelengths to be transmitted. Which of these prevails
to give the perceived hue depends on the spectral balance of the illumination?
Fine-quality alexandrite has a green to bluish-green color in daylight
(relatively blue illumination of high color temperature), changing to a red to
purplish-red color in incandescent light (relatively yellow illumination).
However, fine-color material is extremely rare. Less-desirable stones may have
daylight colors of yellowish-green and incandescent colors of brownish red.
Cymophane is popularly known as "cat's eye". This
variety exhibits pleasing chatoyancy or opalescence that reminds one of the eyes
of a cat. When cut to produce a cabochon, the mineral forms a light-green
specimen with a silky band of light extending across the surface of the stone.