It
crystallizes in the tetragonal system, is prismatic in habit and has
pyramidal terminations. It may be combined with one or other or both of
the square prisms on and a -which are at 45°. Cleavage
is prismatic and pyramidal, but imperfect. Fracture: Conchoidal.
Specific gravity = 4-0 to 4-80. Hardness = 7-1/2. Colour: Brown,
brownish red and gray, and is due to iron and other constituents.
Transparent to opaque and has adamantine lustre. Refractive index = 1-8
to 2-0, more than that of corundum and less than that of diamond.
Double refraction is 0-06, and dispersion 0-048. Uniaxial and
positive. Impurities are iron and hafnium. Composition: ZrSiO4.
Some
zircons show some anomalous properties; for instance, some crystals are
biaxial, some are uniaxial and some isotropic. These three characters
may be shown by different portions of the same crystal, with sometimes
division into separate zones. Colours show a wide range and are liable
to change on exposure to sunlight, heat> X-rays, etc. Some zircons
change colour on heating or may be completely decolourized. Much of the
colourless zircon used as gems has been so treated; with its brilliant
lustre and high dispersion it resembles diamond to some extent and is
known as ' Matura diamond', from Matura in Ceylon. The sky-blue zircon
may also be heat treated.
Zircon has also been divided into three qualities. A zircon : sp. gr. = 4-0; refractive index = 1-80 to 1-89; and birefringence 0-01. B zircon:
sp. gr. =4-70;* refractive index = 1-92 to 1-99; and birefringence =
0-05 to 0-06. These two zircons are not changed when heated, r zircon, which has values intermediate between those of A and B zircons, when heated change into B zircon. All these forms may form part of the same crystal.
Uses
Some of the colourless and coloured varieties are used as gem-stones. When subjected to heat treatment, they
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