Information on Zircon gemstone

by Manisha G

Zircon:

Zircon-gemstones-types

Chemical composition:

Silicate of Zirconium. ZrSiO4 with traces of Uranium and Thorium.

Crystal Characteristics:

Tetragonal system. Prismatic habit, occurs in square prisms terminated at either end by four sided pyramids. Gem uality zircon is nearly always found as water worn crystals in sand and gravels.

Varieties:

Sri Lanka stones can be red, purple, brown, yelloww or green. Most of the blue golden and colourless types owe their colour to heat-treatment. “Jargoon” is an old fashioned term chiefly applied to yellow or colourless zircon. The name “zircon” with suitable colour prefix, is to be prefered and thhe obsolete names should not be used.

Physical Properties:

There are two distinct types, with very different properties, and other stones may cover any part of range between these types.

1. The normal or “high” has hardness 7 – 7 ½; S.G. 4.67 to 4.70; R.I. 1.925 to 1.984 with birefringence 0.059, uniaxial, positive. All “fired” zircons are produced by heating red-brown high type stones.

2. The Metamic “low” type are found principally in Sri Lanka. It has hardness 6; S.G. 3.90 to 4.10, R.I. 1.790 to 1.810, with little or no birefringence. In thhis type, which is mostly green in colour, internal decomposition has taken place into amorphous SiO2 and amorphous or possibly monoclinic ZrO2. This is due to the effect on the crystal lattice (over millions of years) of the small amount of radio active elements present. Intermediate types, in which S.G., R.I. and D.R. vary between low and normal, are common in Sri Lanka. Strong prolonged heating will cause most of the lower types to show a tendency to revert to the fully crystalline normal zircon. This usually spoils the colour and is not a commercially useful process.

Dichroism is very feeble exceppt in blue zircon, in which it is quite strong.

Other important features:

Many zircons have outstanding absorption spectra consisting of narrow dark lines distributed throughout the sppectrum. “Fired” and other normal type may show only the most persistent of these as a faint line in the orange-red at 6535 A, but this is sufficient to identify the stone. The low and intermediate types reveal their unusual nature in their absorption spectra. The spectrum for normal zircon consists of sharply defined narrow lines. The low type has the same lines but they are generally thick and fuzzy in outline. Intermediate types gradually approach the sharpness of normal zircon.

The spectrum of zircon is at once quite characteristic and most variable. It may consists of the single “zircon line” at 6535 A, very thin and faint or may show any thing upto 70 lines varying greatly in numbers.

Cut:

Brilliant (often modified by 8 extra facets round the culet), step, mixed and irregular native cuts from Sri Lanka.

Detection:

The detection of zircon is not generally difficult. The refractometer will give a negative reading in ppractically all cases. Birefringence will show as doubling of the back facet edges and the spectroscope will in almost all the cases show the quite distinctive spectrum. Even if the latter is reduced to the single “zircon line” at 6535 A, it is quite easily recognised.

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