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Alexandrite

Tsarstone collectors guide

Chrysoberyl Classification and Properties

"Crude classifications and false generalizations are the curse of organized life." George Bernard Shaw

Chrysoberyl is the third hardest naturally occurring gemstone and lies between corundum and topaz on the hardness scale. Only corundum and diamond are harder and chrysoberyl is well known for its toughness and durability. Rubbing chrysoberyl will produce a static charge that is retained for several hours. Chrysoberyl is a mineral consisting of ordinary colorless or yellow transparent chrysoberyl, cymophane (chrysoberyl cat´s eye), and alexandrite.

Ordinary chrysoberyl is a yellowish-green, transparent to translucent and often has been referred to in the literature as chrysolite due to the common olive color of many of its gems, but that name is no longer used in the gemological nomenclature. When the mineral exhibits good pale green to yellow color and is transparent, then it is used as a gemstone. Chrysoberyl is often found as twins that produce wedge-shaped or heart-shaped crystals with v-striations. It is also found in pseudohexagonal crystals with repeated twining resulting in sixlings and less commonly as contact twins that are extended along the c-axis (c[001]). Also found as granular to massive material.

Chrysolite
Chrysolite
Fig. 12.: Ordinary chrysoberyl is yellowish-green, transparent to translucent and often has been referred to in literature as chrysolite, but that name is no longer used in the gemological nomenclature. 12

Alexandrite, a strongly pleochroic (dichroic) gem, will exhibit emerald green, red and orange-yellow colors and tend to change color in artificial light compared to daylight. The color change from red to green is due to strong absorption of light in the yellow and blue portions of the spectrum. Typically, alexandrite has an emerald green color in daylight but exhibit a raspberry red color in incandescent light. Only chrysoberyl displaying a distinct change of color should be designated as alexandrite. This means that chrysoberyl and alexandrite can be difficult to differentiate. Since stones with a weak change may be called alexandrite or chrysoberyl, the determination can be subjective. As the color change in alexandrite is due to the presence of chromium and the color of yellow or brown in ordinary chrysoberyl is due to the presence of iron, spectroscopic examination will reveal these differences and help with identification. Chrysoberyl usually shows no fluorescence.

Cymophane is popularly known as cat´s eye. This variety exhibits pleasing chatoyancy or opalescence that reminds one of an eye 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. The finest quality cat´s-eye has a sharp silvery white line across the stone that appears to open and close as the stone is rotated and exhibits a strong "milk and honey" effect (the stone on one side of the eye appears lighter than the other).

Chrysoberyl properties datasheet
SynonymsAlumoberyl, Chrysberil, Chrysoberil, Chrysopal, Delamétherie, Crisoberilo, Krysoberril, Oriental Chrysolite Pacific Cat´s Eye, Cat´s eye, cat eye.
VarietiesAlexandrite, Cymophane
IMA statusValid Species (Pre-IMA)
Identifying characteristicsFingerprint inclusions, silk, color change
Absorption spectraTwo strong lines at 680.5 and 678.5 nm and weak lines at 665, 655, and 645 nm, partial absorption between 580 and 630, three weak lines at 476.5, 473, and 468 nm, and general absorption in the violet.
BirefringenceWeak (0.008-0.011)
Chemical compositionCr3+: BeAl2O4
ClassOxides
CleavageDistinct on {011} perfect, indistinct on {010}, and poor on {100}
ColorGreen in daylight and red-violet in incandescent light.
Crystal habitPrismatic crystals are elongated on c, and often tabular on {001}, with striations parallel to [100]
Crystal systemOrthorhombic. 2/m2/m2/m
DensityG/cm3 3,79
Dispersion.015
Hardness8.5
Heat sensitiveNo
Lattice constantsA a=5.47, b=9.39, c=4.42
LustreVitreous to greasy
Melting point1870 C
Molecular weight126.97 gm
Beryllium 7.10 % Be 19.70 %
BeO
Aluminum 42.50 % Al
80.30 % Al2O3
Oxygen 50.40 % O
Optic characterDR, biaxial positive
Optical effectsMetamerism, pleochroism
PleochroismStrong, X= red-purple, Y= orange, Z= green
Refractive index1.745 - 1.759 (+ .004, - .006)
Specific gravity3.70 to 3.78
StabilityHeat tolerance: Stable
Light tolerance: Stable
Chemical tolerance: None
StreakWhite
ToughnessExcellent
TransparencyTransparent to translucent
TwinningCommonly twinned on {130} resulting in a triangular or kite shaped form, repeated cyclic twinning produces pseudo hexagonal trillings.
Ultraviolet fluorescenceInert to moderate red (LW and SW)

The mineral chrysoberyl is very rare but periodically found as an accessory mineral in granite pegmatites, aplites and mica schists. It is also described in altered zones around the margins of ultramafic rocks where they have been intruded by Be-rich granite pegmatites. Because of its favorable hardness and relatively high specific gravity it may also be found as a placer mineral downstream from these hosts. Chrysoberyl has high enough specific gravity that it will concentrate with black sands in active or paleoplacer stream deposits and concentrate with other relatively heavy minerals such as cassiterite, diamond, corundum, topaz and garnet. When found in placers, it will have rounded edges instead of sharp, wedge-shape forms. Much of the chrysoberyl mined in Brazil and Sri Lanka is recovered from placers as the host rocks have been intensely weathered and eroded.

Chrysoberyl deposits can be divided into three types, excluding rare dolomitic hosts. These include chrysoberyl in pegmatites intruded into ultramafic rocks, chrysoberyl hosted by pegmatites intruded into aluminous rocks, and chrysoberyl found as a primary mineral in REE-pegmatites.

There have been very few research projects on the genesis of chrysoberyl due to its rarity in primary host rocks since most chrysoberyl is recovered from placers. However, it may be hypothesized that in order to produce chrysoberyl, metamorphic overprint of some beryllium- and aluminum-rich pegmatites may be necessary.


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Bibliographic details
Page title Chrysoberyl Classification and Properties
Author David Weinberg
Website title Alexandrite Tsarstone Collectors Guide
Date published 03 July 2010 05:40 UTC
Date accessed 24 April 2014 22:14 UTC
Permanent link http://www.alexandrite.net/viewpage.html?id=ALXS-002-00007
AMA style
David Weinberg. "Chrysoberyl Classification and Properties". In Alexandrite Tsarstone Collectors Guide. July 03, 2010, 05:40 UTC. Available at: http://www.alexandrite.net/viewpage.html?id=ALXS-002-00007. Accessed April 24, 2014.
APA style
"Chrysoberyl Classification and Properties". (2010, July 03). In Alexandrite Tsarstone Collectors Guide. Retrieved 22:14, April 24, 2014, from http://www.alexandrite.net/viewpage.html?id=ALXS-002-00007.
Bluebook style
"Chrysoberyl Classification and Properties", http://www.alexandrite.net/viewpage.html?id=ALXS-002-00007 (last visited April 24, 2014).
Bluebook: Harvard JOLT style
See Alexandrite Tsarstone Collectors Guide, "Chrysoberyl Classification and Properties", http://www.alexandrite.net/viewpage.html?id=ALXS-002-00007 (A comprehensive guide to the chrysoberyl family and it's varieties, including alexandrite and cymophane gemstones) (as of Apr. 24, 2014, 22:14 GMT).
CBE/CSE style
David Weinberg. "Chrysoberyl Classification and Properties". In Alexandrite Tsarstone Collectors Guide. 2010 Jul 03, 05:40 UTC.[Internet], Alexandrite Tsarstone Collectors Guide; 2010 Jul 03, 05:40 UTC [cited 2014 Apr 24]. Available from: http://www.alexandrite.net/viewpage.html?id=ALXS-002-00007.
Chicago style
David Weinberg, "Chrysoberyl Classification and Properties", http://www.alexandrite.net/viewpage.html?id=ALXS-002-00007, (accessed April 24, 2014).
MLA style
"Chrysoberyl Classification and Properties". Alexandrite Tsarstone Collectors Guide. 03 Jul 2010, 05:40 UTC. 24 Apr 2014 http://www.alexandrite.net/viewpage.html?id=ALXS-002-00007
MHRA style
David Weinberg, "Chrysoberyl Classification and Properties", Alexandrite Tsarstone Collectors Guide, 03 July 2010, 05:40 UTC, http://www.alexandrite.net/viewpage.html?id=ALXS-002-00007 [accessed 24 April 2014].
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