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Ruby
The most valuable variety of corundum is ruby.
The name comes from the Latin rubrum, "red. Like other
red stones, it has also been called carbunculus, or carbuncle,
meaning a small coal or ember
.
Appearance The color varies from fiery vermilion
to violet red, but because rubies are pleochroic, different
colors are also found in the same stone; bright or sometimes
brick -ed in one direction, tending to carmine in the other:
The color is also accompanied by marked fluorescence which
is stimulated by ordinary, artificial light and above all,
by the ultraviolet rays of direct sunlight. Thus rubies turn
brighter red under such light and the purplish ones look "redder.
If the color is too pale, they are no longer called rubies,
but pink sapphires; if it is more violet than red, they are
known as violet sapphires.
But it is hard to establish precise limits,
as all the intermediate shades are possible. The brightest
red and thus the most valuable rubies (usually from Burma)
often have areas full of inclusions in the form of minute
rutile needles (or straws), which interfere with the light,
producing a distinctive silky sheen known, in fact, as silk.
When the silk is not heavy, the stones are clearer, more attractive,
and even more valuable.
Other, mainly crystalline inclusions are normally
found as well. Rubies of this type are not usually more than
a few carats in weight. The rare exceptions generally contain
copious inclusions. Violet red, sometimes quite dark, rubies
come principally from Thailand. The type most often found
on the market nowadays, they can be several carats in weight.
They are normally clearer, without patches of silk.
While good-sized clear stones are found, specimens
with many inclusions are commonly sold as well. Rubies are
usually given a mixed cut, which is generally oval, but can
be round or, more rarely, other shapes. In the past, they
were given a cabochon cut, like all stones outstanding for
their color. Today, however, this cut is reserved for less
transparent stones with numerous inclusions.
Distinctive features Rubies
can often be distinguished by their immediately visible characteristics:
a fairly obvious pleochroism, a distinct brightening of color
in strong light, the silk effect (where present), and a considerable
luster. While spinel can be a similar color and has a similar
luster, it is not pleochroic, turns much less bright in strong
light, and never displays the silk effect. Red garnet is not
pleochroic and the color does not brighten in strong light;
it has a similar luster, but when given a faceted cut often
displays dark, blackish areas within the stone.
Red tourmaline is usually a completely different
shade, but can be very similar, with a pleochroism comparable
to that of ruby. It does not, however, brighten in strong
light, and this can be sufficient indication to warrant testing
its physical properties, which are quite different. The other
red gemstones mentioned also differ physically from ruby.
Some caution is needed with garnets, which show wide variations
in both density, which in some cases coincides with that of
corundum, and refractive index, which can coincide with one
of the figures for corundum. Garnet; however, is singly refractive,
and examples with an index in the region of 1 .76 have a lower
density than that of ruby.
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