Physical Properties
|
| Chemical Composition |
Beryllium aluminate, expressed by the formula BeAIZ04. |
| Crystallographic Character |
Orthorhombic system. Habit; prismatic crystals. Cyclic, or radial, twinning and striations are frequent. |
| Hardness |
8 ½ |
| Toughness |
Excellent, except some nonphenomenal material that has been reported as only fair to good.
|
| Cleavage |
Indistinct |
| Fracture |
Conchoidal |
| Specific Gravity |
3.71 to 3.75; normal 3.73 |
| Streak |
White |
| Characteristic Inclusions |
Chrysoberyl has no inclusions that can
be regarded as characteristic of a significant proportion of specimens. Finger print inclusions, similar to those in corundum, are not uncommon. Silk like inclusions (negative crystals or rutile crystals), which cause the chatoyant band in cat's-eye, are also frequently encountered; however, they occur in a single direction, rather than in three sets of parallel needles, as in corundum, and are usually exceedingly fine. Yellow transparent stones only frequently show step like glide planes or twinning lines.
|
Optical Properties
|
| Degree of Transparency
|
Transparent to translucent |
| Luster |
Polished surfaces are vitreous to sub-adamantine: fracture surfaces are vitreous to greasy |
| Refractive Index |
1.746-1.755. These are the common readings. Each may vary about .004, but the birefringence is always. 008 to .009. |
| Birefringence |
.008to .009 |
| Optic Character |
Biaxial positive |
| Pleochroism |
Yellow: distinct vary light yellow, greenish yellow, and colorless. Alexandrite: strong green, orange, and dark red purple |
| Dispersion |
0.015 |
| Phenomena |
Chatoyancy in cat's-eve and change of color in alexandrite. Four-rayed stars are known to occur, but they are exceedingly rare. |
| X-Ray Fluorescence
|
Alexandrite may show a weak red glow; all others are inert. |
| Transparency to X-Rays |
Semitransparent |
| Ultraviolet Fluorescence |
Alexandrite may show a weak red glow under both long and short wavelengths. Exposure to short-wave ultraviolet light produces a yellow-green glow in greenish yellow chrysoberyl . All others are inert. |
| Color-Filter Reaction |
Alexandrite: red; brighter in artificial light. Yellow and green: no reaction. |
| Absorption Spectra |
Alexandrite: a strong doublet at 6805 and 6785 A.U. and weak lines at 6650, 6550 and 6450 A.U. Partial
absorption
of the yellow between 5800 and 6300, as well as three weak lines in the blue at 4765, 4730 and 4680 A.U. Also, general absorption in the violet. Yellow-green:
a strong band in the bluish violet at 4450 A.U. The spectrum in alexandrite is caused by chromium; in the yellow green type, by ferric iron.
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