Biaxial
A double-refractive gemstone, which has two directions of single refraction or optic axes.
Birefringence
The numerical measurement of double refraction in gemstones.
Biaxial
The character of crystals from the monoclinic, rhombic, triclinic system which show double refraction but have two direction of single refraction.
Binding
Two or more metal part are fixed together with a metal wire, for soldering.
Biaxial...Two optic axes or double refraction. Usually crystals in the rhombic, monoclinic, and triclinic system.
Bipyramid (dipyramid)...Crystals that form symmetrically about a plane dividing it into two pyramids.
Biaxial (positive or negative; the beta index is usually near halfway between alpha and gamma)
Crystal System
Orthorhombic; usually occurs as rounded pebbles; well formed crystals are quite rare.
Biaxial Gemstones (a, n) Biaxial gemstones have two optical axes that do not line up to the c-axis. In uniaxial stones, the optic axis is parallel to the c-axis.
Idocrase is double refractive and can exhibit positive or negative uniaxial optic character and sometimes anomalous positive or negative biaxial characteristics. Refractive indices of crystals found in serpentinites are highest with 1.705-1.750 and 1.
If χxx ≠ χyy and χxx ≠ χzz the crystal is called biaxial. A uniaxial crystal exhibits two refractive indices, an "ordinary" index (no) for light polarised in the x or y directions, ...
Dichroism is found in uniaxial crystals with single optic axis and trichorism is found in biaxial crystals with two optic axis.
Optics -- R.I 1.74-1.77, biaxial positive or negative.
Durability -- Hardness 8.5. Chrysoberyl is also exceptionally tough (resistant to breakage), so it produces some extremely durable gems.
Crystal structure -- Orthorhombic.
Uniaxial crystals can potentially show two colors, while biaxial crystals may show up to three colors.
OPTIC CHARACTER: The nature of a gem as either uniaxial (singly refractive) or biaxial (doubly refractive).
ORIENT: The iridescent luster of a pearl.
Uniaxial Gemstones
In uniaxial stones, the optic axis is parallel to the c-axis. Biaxial gemstones have two optical axes that do not line up with the c-axis.
Those species with a single optic axis are known as "uniaxial" and those with two are, logically, called "biaxial".
The hardness of this stone is 6 to 6.5 on Mohs scale. The crystal structure of the stone is triclinic and the refractive index ranges from 1.54 to 1.55. The optical character of this stone is biaxial with the positive optic sign.
 See also: Color, Crystal, Stone, Crystals, Gemstone
  
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