Frederick William Rudler

“ The effect of heat on zircon is remarkable. Most coloured zircons, exposed to a high temperature, either change or lose their colour, but this loss is attended by a gain in brilliancy. The “Matura diamonds” of Ceylon are zircons which have been thus artificially decolorized. Certain zircons when heated in a Bunsen-flame glow with an orange incandescence, whilst others may emit an orange glow when ground on a copper-wheel fed with diamond-dust. Even exposure to sunlight will sometimes modify the colour and lustre of a zircon. ”

“ It is notable that the specific gravity has a very wide range, extending from a little below 4 to rather more than 4·7, and being thus greater than that of any other gem-stone. Rarely colourless, zircon is usually brown or red, sometimes orange, yellow or green, and occasionally parti-coloured or zoned.
Whilst common zircon is opaque, the gem-varieties are transparent. The dichroism of coloured zircons is always feeble; the double refraction usually strong and of positive sign; and the optical properties of some zircons suggest a biaxial mineral. ”