PLEOCHROISM
Gemstones such as ruby or blue sapphire that are doubly refractive
(having the ability to split a beam of light entering the gemstone
into two rays) may show two or three different colors when viewed
from different directions. Iolite is a remarkable example of this.
As shown here, a deep blue is seen when viewed from one direction,
while a second direction is nearly colorless with a very slight brownish
tint, and the third direction also appears nearly colorless, this
time with a very slight bluish hue.
A clearly defined separation of colors such as this depends on the
orientation of the cut in relation to the crystal, and even a slight
difference in angles will have a pronounced effect on the beauty
of the gemstone. The characteristic calm beauty of iolite is the
result of the crystal's inherent pleochroism combined with an appropriate
cut to bring it out. Tanzanite, which has been especially popular
in the United States over the past decade, is another gemstone with
strong pleochroism. |
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JUDGING QUALITY
Gem-quality iolite is highly transparent with a beautiful violet-blue color,
falling in the range of beauty grades S to A and tone levels 6 to 4 on
the quality scale. It is apparent that some of the stones in column D are
milky and have low transparency. These are accessory quality, and will
usually be cut as cabochons. Such milky cabochons exhibit a soft beauty,
and are used in affordable adornments because of their low prices. |
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