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A variety of gemstones of which the common natural colour is reddish-brown but stones exhibiting green and several other natural colours are found. However, most marketed specimens are brown stones that have been subjected to heat treatment which produces a colourless stone or stones of a wide range of colours, especially blue, bluish-green, purple, deep red, and golden-yellow.
When the stones are heated in a closed container, they become colourless or blue; when a flow of air is permitted to enter the container, they become golden-yellow or red. Such converted colours are fairly stable, but sometimes revert in time to greenish- or brownish-blue. Certain coloured varieties have been given special names (e.g. hyacinth, jacinth, jargoon and starlite) which have been recommended to be discarded in favour of merely prefixes of the particular colour.
The zircon has adamantine lustre and high colour dispersion, so that a colourless stone often resembles a diamond, but with less fire and brilliance. All zircons are brittle and tend to chip at the facet edges. They are classified as 'high' or 'normal' (which are crystalline), 'low' or metamict (which are amorphous or nearly so), and intermediate (which can be converted by heat to 'high').
Zircons are faceted usually in the mixed cut or zircon cut style. Most zircons are treated and cut in Bangkok, Thailand. Misnomers that have been applied to zircon are 'Siam aquamarine', 'Matara diamond', and 'Ceylon diamond'. Synthetic zircons have been produced but not commercially; a blue synthetic spinel has been miscalled a 'synthetic zircon'.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson