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YAG
This is the name given to the more-or-less yellow
variety of corundum, in accordance with modern terminology
for colors other than blue and red. It was also formerly known
as "oriental topaz."
Appearance Yellow sapphire
may occur in quite a wide range of colors, from pale to canary
yellow, gold, honey, and brownish yellow. The lighter, brighter
colors are the most common. Medium-sized or large stones are
often seen, generally with a mixed oval cut having a rather
large pavilion (to increase the depth of color). Because of
their transparency, veillike or lacelike liquid inclusions
and even foreign crystals are often visible inside these stones.
Like all corundum, they have considerable luster. Honey-colored
stones, which are less common and often smaller, are equally
lustrous and are given both oval and rectangular step cuts.
The color can be quite similar to that of certain topazes.
Distinctive features Like all
corundum, the yellow type can often be distinguished from
other gems by its luster, but not from yellow chrysoberyl,
which is very similar in this respect. The canary yellow color
is not often found in other stones, except citrine, which
is, however, a bit less lustrous. Yellow zircon has luster
similar to that of yellow sapphire but is usually distinguishable
through a lens by its much stronger birefringence, the facet
edges appearing much more clearly duplicated and both facets
and edges giving an impression of less hardness and greater
brittleness. The inclusions in yellow sapphires are also fairly
characteristic, at least when viewed through a lens. All in
all, however, many yellow gems show some resemblance to yellow
sapphire and must, therefore, be distinguished by their physical
properties.
Occurrence Most light or canary
yellow sapphire comes from Sri Lanka, and only a small proportion
from Australia, which, however, supplies most of the less
common honey, golden yellow or yellow-brown stones. Limited
quantities are obtained from Burma.
Value Somewhat surprisingly,
even very attractive yellow sapphires with fine luster are
of relatively low value compared with other secondary gems;
they are, for example, worth a lot less than the'pink variety.
Simulants and synthetics Yellow sapphire
was never really imitated by other substances until synthetic
corundum was first produced. Since then canary and honey-yellow
varieties have been widely manufactured. Like pink and colorless
synthetic corundum, the yellow variety produced by the Verneuil
flame fusion method is highly convincing and harder to distinguish
from its natural counterparts than are rubies and sapphires
produced in this way.
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