Physical Properties
|
| Chemical Composition
|
Aluminum oxide, expressed by the formula
Al2O3. The color is caused by chromium
Oxide in the red, titanium and iron oxide in the blue, iron
oxide in the yellow, chromium and iron oxide in the orange,
iron and titanium oxide in the green, and chromium, titanium
and iron oxide in the purple. |
| Crystallographic Character |
Hexagonal system. Habit: ruby usually crystallizes
in six-sided prisms, terminated by flat faces (basal pinacoids)
as shown in the photograph. Sapphire (except Montana) usually
occurs in a double pyramid with twelve inclined faces, often
modified by several extra sets of six above and below (sketch).
The inclination of the pyramidal faces may be so slight that
the crystals resemble simple hexagonal prisms. |
| Hardness |
9 |
| Toughness |
Excellent, except in laminated (repeatedly
twinned) or fractured stones. Many star corundums contain fractures
or incipient fractures that may be extended by blows delivered
during ordinary wear. |
| Cleavage |
None. Parting, or false cleavage, often occurs
due to twinning parallel to the base or a rhombohedral direction.
Twinning is particularly common in black star sapphires, which
is the reason for their usual shallow cut. |
| Fracture |
Conchoidal |
| Specific Gravity |
Ruby: 3.95 to 4.05 normally 4.00; Sapphire:
3.95 to 4.03 normally 3.99. |
| Streak |
White |
| Characteristic Inclusions |
"Silk", which is common in both ruby and
sapphire, occurs in two forms: needlelike rutile crystals or
long, narrow negative crystals, both of which are arranged in
three sets of parallel threads that intersect one another at
sixty-degree angles. Other crystal inclusions include zircon,
usually surrounded by a halo of black fractures, tiny spinel
octahedral, mica, hematite slabs, rounded grains of garnet,
coarse rutile needles, and corundum crystals and grains of low
liquid and gas filled inclusions arranged in a "fingerprint"
pattern. Prominent hexagonal growth lines and color zoning are
common in both varieties of corundum. |
Optical Properties
|
| Degree of Transparency
|
Transparent to opaque. |
| Luster |
Fracture surfaces are vitreous; polished
surfaces are vitreous to sub adamantine. |
| Refractive Index |
1.762-1.770. Green sapphire may be about
0.01 higher. |
| Birefringence |
0.008 |
| Optic Character |
Uniaxial negative |
| Pleochroism |
Ruby: strong purplish red and orangey red.
Blue sapphire: strong violetish blue and greenish blue. Green:
strong green and yellow-green. Yellow: weak to distinct yellow
and light yellow. Orange: strong yellow-brown or orange and
colorless. Purple: strong violet and orange. |
| Dispersion |
0.018 |
| Phenomena |
Asterism, Very rarely a cat's-eye effect
is encountered; also, an alexandrine color change from blue
to purple and very rarely from green to reddish brown. |
| X-Ray Fluorescence |
Burma and Ceylon rubies: strong red. Siam
ruby: weak red. Blue sapphire: usually inert, but may show a
weak red glow. Green: inert. Yellow usually inert, but may show
a weak orange-yellow glow. Orange: strong red. Colorless: moderate
red. Violet: strong red. |
| Transparency to X-Rays
|
Nearly transparent. |
| Ultraviolet Fluorescence |
Burma ruby: strong red (long wavelengths;
moderate red (short wavelengths). Ceylon ruby: strong orange-red
(long wave lengths; moderate orange-red (short wave lengths).
Siam ruby: weak red long wavelengths): weak red to none (short
wavelengths). Pink sapphire: strong orange-red (long wavelengths;
very tight sapphire: strong orange-red (long wavelengths). Ceylon
yellow (golden) sapphire: moderate orangey yellow (strength
is in direct relations hip to depth of color in long wavelengths);
weak yellow-orange (short wavelengths). Green sapphire: none.
Ceylon light blue sapphire: strong orange to red (long wavelengths).
Other blue stones are virtue UV inert, with the exception of
some Siam stones that fluoresce greenish white under short wavelengths,
thus resembling synthetic sapphires under the same conditions.
Violet and alexandrite like sapphire: strong red (long wavelengths):
weak light red (short wavelengths). Colorless sapphire: moderate
light orange-red. |
| Color-Filter Reaction
|
Ruby: strong red. Green sapphire: green.
Blue sapphire: blackish, Purple and purplish-blue stones; may
appear reddish. |
| Absorption Spectra |
Ruby: a strong doublet at 6942 and 6928 A.U.;
which may appear as a fluorescent line; fairly distinct lines
at 6680 and 6595 A.U. , broad absorption from 6200 to 5400 A.U.,
a strong doublet at 4765 and 4750 A.U., a weak line at 4685
A.U., and general absorption of the violet. Blue sapphire: three
bands in the blue at approximately 4500, 4600 and 4700 A.U.
In Australian stones all three bands are usually distinct, but
only the 4500 line is usually visible in those from Ceylon.
Green: same three bands but somewhat stronger. Yellow (Australian):
same three bands but usually weaker. Yellow stones from other
sources show no typical absorption spectrum. Purple stones may
show a combination of the ruby spectrum (chromium) and sapphire
spectrum (iron). |
