Prices of rubies are primarily determined by color. The brightest and
most valuable "red" called blood-red, commands a large premium over
other rubies of similar quality. After color follows clarity: similar to
diamonds, a clear stone will command a premium, but a ruby without any
needle-like
rutile inclusions may indicate that the stone has been treated. Cut and
carat (weight) are also an important factor in determining the price.
Rubies have a
hardness of 9.0 on the
Mohs scale of mineral hardness. Among the natural gems only
moissanite and
diamond
are harder, with diamond having a Mohs hardness of 10.0 and moissonite
falling somewhere in between corundum (ruby) and diamond in hardness.
Ruby is α-alumina (the most stable form of Al
2O
3) in which a small fraction of the aluminium
3+ ions are replaced by chromium
3+ ions. Each Cr
3+ is surrounded octahedrally by six O
2- ions. This crystallographic arrangement strongly affects each Cr
3+,
resulting in light absorption in the yellow-green region of the
spectrum and thus in the red color of the gem. When yellow-green light
is absorbed by Cr
3+, it is re-emitted as red
luminescence.
[2]
This red emission adds to the red color perceived by the subtraction of
green and violet light from white light, and adds luster to the gem's
appearance. When the optical arrangement is such that the emission is
stimulated by 694-nanometer photons reflecting back and forth between
two mirrors, the emission grows strongly in intensity. This effect was
used by
Theodore Maiman in 1960 to make the first successful
laser, based on ruby.
All natural rubies have imperfections in them, including color impurities and inclusions of
rutile
needles known as "silk". Gemologists use these needle inclusions found
in natural rubies to distinguish them from synthetics, simulants, or
substitutes. Usually the rough stone is heated before cutting. Almost
all rubies today are treated in some form, with heat treatment being the
most common practice. However, rubies that are completely untreated but
still of excellent quality command a large premium.
Some rubies show a three-point or six-point
asterism or "star". These rubies are cut into
cabochons
to display the effect properly. Asterisms are best visible with a
single-light source, and move across the stone as the light moves or the
stone is rotated. Such effects occur when light is reflected off the
"silk" (the structurally oriented
rutile
needle inclusions) in a certain way. This is one example where
inclusions increase the value of a gemstone. Furthermore, rubies can
show color changes—though this occurs very rarely—as well as
chatoyancy or the "cat's eye" effect.
Factors Affecting Values
Diamonds are graded using criteria that have become known as the four
Cs, namely color, cut, clarity and carat weight. Similarly natural
rubies can be evaluated using the four Cs together with their size and
geographic origin.
Color: In the evaluation of colored gemstones, color is the most important factor. Color divides into three components;
hue,
saturation and
tone.
Hue refers to "color" as we normally use the term. Transparent
gemstones occur in the following primary hues: red, orange, yellow,
green, blue, violet. These are known as
pure spectral hues.
[11]
In nature there are rarely pure hues so when speaking of the hue of a
gemstone we speak of primary and secondary and sometimes tertiary hues.
In ruby the primary hue must be red. All other hues of the gem species
corundum are called sapphire. Ruby may exhibit a range of secondary
hues. Orange, purple, violet and pink are possible.
The finest ruby is best described as being a vivid medium-dark toned
red. Secondary hues add an additional complication. Pink, orange, and
purple are the normal secondary hues in ruby. Of the three, purple is
preferred because, firstly, the purple reinforces the red making it
appear richer.
[11]
Secondly, purple occupies a position on the color wheel halfway between
red and blue. In Burma where the term pigeon blood originated, rubies
are set in pure gold. Pure gold is itself a highly saturated yellow. Set
a purplish-red ruby in yellow and the yellow neutralizes its complement
blue leaving the stone appearing to be pure red in the setting. (
wikipedia.org)