Chrysoberyl

Chrysoberyl
Cyclic trilling of chrysoberyl
General
Category	Oxide minerals
Formula (repeating unit)	BeAl2O4
IMA symbol	Cbrl[1]
Strunz classification	4.BA.05
Crystal system	Orthorhombic
Crystal class	Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m)
Space group	Pbnm
Unit cell	a = 5.481 Å, b = 9.415 Å, c = 4.428 Å; Z = 4
Identification
Color	Various shades of green, emerald-green yellow, blue, brownish to greenish black, may be raspberry-red under incandescent light when chromian; colorless, pale shades of yellow, green, or red in transmitted light
Crystal habit	Crystals tabular or short prismatic, prominently striated
Twinning	Contact and penetration twins common, often repeated forming rosette structures
Cleavage	Distinct on {110}, imperfect on {010}, poor on {001}
Fracture	Conchoidal to uneven
Tenacity	Brittle
Mohs scale hardness	8.5
Luster	Vitreous
Streak	White
Specific gravity	3.5–3.84
Optical properties	Biaxial (+)
Refractive index	nα=1.745 nβ=1.748 nγ=1.754
Pleochroism	X = red; Y = yellow-orange; Z = emerald-green
2V angle	Measured: 70°
References	[2][3][4][5]
Major varieties
Alexandrite	Color change; green to red
Cymophane	Chatoyant

The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula BeAl₂O₄.^[5][6] The name chrysoberyl is derived from the Greek words χρυσός chrysos and βήρυλλος beryllos, 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).^[7]

An interesting feature of its crystals 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" 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 results.

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 polarised 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).^[8] However, fine-color material is extremely rare. Less-desirable stones may have daylight colors of yellowish-green and incandescent colors of brownish red.^[8]

Cymophane is popularly known as "cat's eye". This variety exhibits pleasing chatoyancy or opalescence that reminds one of the eye of a cat.^[9] 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.