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Cordierite (or water sapphire)
Gem quality cordierite has the same name (or
rather names) as the mineral itself, but was in the past also
known as water sapphire, particularly when the color was not
very intense. This is perhaps its most familiar name in the
trade.
Appearance The color may vary
from quite a deep blue to violet blue, light blue, or grayish
blue; but it always has very strong pleochroism, being a much
lighter gray or wan yellow in one direction. For obvious,
aesthetic reasons gems are cut so that this color is only
visible from the side. The most common type of cut is the
rectangular, step type not always with truncated corners.
One also comes across cordierites with a cabochon cut or minutely
engraved, particularly in the case of less transparent specimens
with numerous inclusions. Most stones are a few carats in
weight: not too small, therefore, but never very large.
Distinctive features An essential characteristic
of cordierite is its exceptional pleochroism, which may, however,
resemble certain tanzanites. Many cordierites have a decidedly
cold, grayish coloration, whereas tanzanite is a warmer color,
always with a hint of violet. Testing the density, which is
very different, would remove any uncertainty.
Occurrence Cordierite comes mainly from the
gem gravels of Sri Lanka, but also from the United States
and Namibia.
Value Rather low, that of minor gemstones.
Simulants and synthetics It is neither imitated
norduced synthetically.
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