Physical Properties
|
| Chemical Composition |
A beryllium-aluminum silicate, expressed by the formula Be2Al2(SiO3)6 |
| Crystallographic Character |
Hexagonal system. Habit: usually, the simple prismatic form, more
perfectly formed than most gem crystals. Crystals are sometimes modified and terminated by basal pinacoids. |
| Hardness |
7 1/2 to 8 |
| Toughness |
Poor. Because it is usually flawed, often heavily, emerald should be given great care in handling, setting and cutting. The toughness of aquamarine and the other beryls is classed as good. |
| Cleavage |
Very difficult; parallel to the basal plane of the crystal. |
| Fracture |
Conchoidal |
| Specific Gravity |
Emerald: 2.67 to 2.75; normal 2.71. Other beryls: 2.67 to 2.84. Morganite is usually 2.82 |
| Streak |
White |
| Characteristic Inclusions |
Colombian emerald are characterized
by three-phase inclusions; i.e. cavities containing a solid, a liquid and a gas. Calcite or pyrite are other minerals frequently found in emerald. The following illustrations show the expected types of inclusions found in beryl materials.
|
Optical Properties
|
| Degree of Transparency
|
Transparent to translucent. |
| Luster |
Polished surface are vitreous; fracture surfaces are vitreous to resinous. |
| Refractive Index |
Usually 1.577-1.583. May vary from a low of 1.565-1.570 to a high of 1.590-1.599. |
| Birefringence |
0.005 to 0.009 |
| Optic Character |
Uniaxial negative. |
| Pleochroism |
Emerald: strong green and blue-green. Aquamarine (blue): weak to distinct blue and light blue. Aquamarine (greenish
blue): weak to distinct blue and blue-green. Golden: weak greenish yellow and yellow. Brown: weak brownish yellow and greenish yellow. Morganite: distinct light red and light violet .
|
| Dispersion |
0.014 |
| Phenomena |
Rarely, poor-quality star stones and cat's-eyes. |
| X-Ray Fluorescence
|
Emerald: very weak, dull red. Aquamarine: none. Colorless: none to
pale yellowish or pinkish. Golden: none. Morganite: an intense crimson glow. |
| Transparency to X-Rays |
Transparent |
| Ultraviolet Fluorescence |
Emerald: none to weak orangey red to violetish red under long wavelength. Aquamarine: none. Colorless: none
to pale yellowish or pinkish under both wavelengths. Golden: none. Morganite: weak, light red under both wavelengths.
|
| Color-Filter Reaction |
Emerald: red. Indian and Transval emeralds are exceptions, however, since they appear green through the filter. Aquamarine and other beryls: none. |
| Absorption Spectra |
Emerald: distinct lines in the red at 6830 and 6805 A.U. Less distinct lines at 6620 and 6460 A.U. Partial absorption of the orange-yellow between 6300 and 5800 A.U. and almost completion absorption of the violet. Aquamarine: An indistinct line in the green and blue at 5370 and 4560A.U., respectively, as well as a strong line in the violet at 4270 A.U., depending on depth
of color of the stone.
|
Effects Caused by:
|
| Heat |
Emerald: Fuses with difficulty before the blowpipe or jewelers torch. May cause additional fracturing or complete breakage of the stone. Heat must be avoided in cleaning, repairing, mounting and cutting. Aquamarine: Whitens and fuses with difficulty before the blowpipe or jewelers torch. The color of aquamarine may be improved
by heating. Some green beryl becomes permanently blue.
|
| Acids |
Emerald and the other beryls are resistant to all acids except hydrofluoric. |
| Irradiation |
When treated with the Van de Graaf generator, beryl acquires permanent coloration. The change is not entirely consistent and most stones are not economically promising, despite the persistence of color. Pale emeralds become somewhat grayish green and unattractive. One light-colored Brazilian specimen, when subjected to prolonged bombardment, became strongly dichroic (and shattered) in bluish violet
and gray-green. Most treated emeralds tend to revert to their original color after
exposure to a few minutes of sunlight, and then do not color so darkly with a second treatment. Some stones retain a little darkening but it is not an improvement, since the green color is no more attractive than before treatment.
A dark aquamarine became quite green when subjected to X-rays, but it was not an emerald green. It retained this color
for several years, until it was heated rather drastically, whereupon it reverted
to the original aquamarine blue. Before electrons the same thing happens: pale aquamarines and colorless beryls turn to pale golden beryls, but the depth of color, which experience has shown to vary from stone to stone, is never intense, and prolonged bombardment does nothing to change it after the first hour or so.
Stones exposed directly to the electron beam tend
to crack badly. Prolonged exposure of the golden stones to sunlight does not
seem to fade them; the color that persists after the first few minutes of exposure to light (and it may even fade somewhat in the dark within minutes of the treatment) resists anything but actual heating to remove it. Research has shown that pale beryls can be made golden in an atomic pile, but they retain the radioactivity. |
