Zircon, because of its high refractive index and high dispersion,
approaches diamond in degree of brilliance and fire. On only casual
examination it is quite possible to mistake a well-cut, colorless zircon
for a diamond. However, a careful examination of the back facets of such
a stone, when viewed through the table, would show strong double
refraction, a characteristic of zircon but not of diamond. Zircon’s
double refraction makes the back facet edges appear doubled. Since
diamond is “singly refracting,” it cannot produce this double appearance
of the back facets.
Zircon is brittle and has a hardness of just over 7, while diamond’s
hardness, as we have seen, is rated at 10. After being worn in jewelry
for a long period of time, zircon will show signs of chipping on the
facet edges. Under the same conditions, diamond would remain unchanged.
Because of this tendency for facet edges to chip, it is the practice in
the gem trade to pack cut zircons separately. If a number of zircons
were placed in the same paper packet there would be a risk of “paper
wear.”
In the gem trade, the most important zircons are those that are
colorless, golden brown, or sky blue. Such stones originally were
reddish brown zircon pebbles from Indochina, but they have been
converted by being subjected to temperatures approaching 1800° F. for
periods of up to two hours. When the original zircons are heated in a
closed container, the stones become blue or colorless; when a flow of
air is allowed through the container, the stones become golden yellow,
red, or colorless. In most of these converted stones the color remains
quite stable, but in some it may revert to an unattractive greenish or
brownish blue after a period of time.
[Illustration: The beautiful colors of these brilliant zircons are
the result of heat treatment given to natural, reddish brown stream
pebbles. The three stones at the left (from top) weigh 118, 103, and
98 carats, and the ones on the right weigh 106 and 29 carats. The
106-carat stone came from Thailand, the others from Indochina.
(Four-fifths actual size.)]
In addition to being reddish brown, natural zircon may vary from almost
colorless to yellow, red, orange, and brown or from yellow-green to dark
green and, occasionally, blue.
The most important producing areas of gem zircon are in a region of
Indochina that comprises parts of Thailand, Viet Nam, and Laos.
Additional gem zircon, like so many of the other gem species, is
recovered from near Moguk in Upper Burma and from the gem gravels of
Ceylon.
There is no synthetic zircon on the market, but a bright blue synthetic
spinel is sometimes used to simulate zircon successfully.