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Sapphire
This is the blue variety of corundum. The name
is probably derived, through the Latin sapphirus and Greek
sapheiros, from a Sanskrit word. As with other gem names,
however, the Latin sapphirus did not originally denote the
gem it is associated with today. Judging by the description
of Pliny the Elder, it almost certainly referred to what is
now known as lapis lazuli, rather than corundum.
Appearance Sapphires can be a very dark blue,
to the point of seeming dense and blackish from a distance,
sometimes accompanied by a blue to dull green pleochroism,
which is only visible from the side in cut stones. They may
also be a strong, but not too bright blue, easily recognizable
from a distance, this being the ideal color. Other possibilities
are light, usually bright, blue, with the color unevenly distributed;
palish blue or, finally, blue with a violet tinge, at least
in bright light. Like all corundum, sapphire always has good
luster.
Some sapphires display clearly defined streaks
of paler color, in contrast to a dark ground. Others have
areas with a slightly silky sheen, which are not clearly delineated.
Still other, uncommon varieties assume a distinct, milky appearance
in strong light, with a marked increase in color intensity.
Inclusions are, as a rule, less obvious in very dark stones,
due to their general lack of transparency, whereas medium
to large, pale stones often show distinct veils" or "feathers"
caused by very fine inclusions and foreign crystals, which
are sometimes transparent, sometimes dark, submetallic, and
opaque, and, very occasionally, bright red. Sapphires are
usually given oval or less frequently, round, mixed cuts,
but rectangular or square, step cuts, with or without trimmed
corners, are also possible. The cabochon cut is used as well,
although less frequently than in the past. Nowadays it is
generally reserved for stones full of inclusions or those
in which the color is concentrated in a few streaks on a light
ground. In the latter case, in fact, the cabochon cut gives
the color a more uniform appearance. Stones weighing several
carats or even 10 to 20 carats in the case of light-colored
specimens, are not uncommon.
Distinctive features Like other
types of corundum, sapphires have a striking luster. The color
is also quite distinctive, whether or not clear blue-green
pleochroism is visible. The overall appearance is very important.
For example, a deep blue color with distinct blue-green pleochroism
and internal streaks straight across or at an angle of 120°,
combined with the powerful luster of corundum, indicates a
sapphire of Australian origin. A slightly patchy, blue color
with imperceptible pleochroism and strong transparency showing
veillike inclusions and a slight silk effect, still with excellent
luster, denotes a sapphire from Sri Lanka. Cornflower to deep
blue in a stone without obvious inclusions but of slightly
milky appearance, acquiring a distinct fullness of color in
bright light, is characteristic of the rare sapphires from
Kashmir. Of the other blue stones, tanzanite always shows
a hint of violet, fairly obvious pleochroism, and less luster
than sapphire. Cordierite, apart from being less lustrous
and violet or gray blue, has striking pleochroism from blue
to an unmistakable drab yellow.
Strongly colored specimens of indicolite tourmaline
are often an attractive greenish blue, with a pleochroism
ranging from blue to green, but the green is very different
from that of sapphire which, when it is present, is always
dull or yellowish. Still on the subject of pleochroism in
tourmaline, the direction corresponding to the blue color
shows a characteristic lack of transparency. While blue zircon
has a luster similar to that of sapphire, it is an electric
blue or blue-green unlike that of any other gemstone.
Furthermore, its strong birefringence, seen
in a clear duplication of the facet edges when viewed through
the stone with a lens, would remove all trace of doubt; sapphire
is doubly refractive as well, but to a much lesser degree.
In the rare cases when blue spinel is not cloudy blue or violet
gray, but a vivid mid-blue, it can look very much like sapphire,
partly on account of its strong luster. In this case, it can
only be distinguished by its physical characteristics; establishment
of single refractivity, or measurement of the density or refractive
index should suffice.
Occurrence The best sapphires were discovered
in a small deposit in Kashmir in 1880, in a remote mountain
area which has now probably been exhausted. Very fine sapphires
are also found in Burma, but in limited quantities.
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