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in sulphuric acid. The acid chars the sugar and the porous layers get different shades of brown and black. This is practised on a large scale in Idar-Oberstein. The best results are obtained with smaller specimens either roughly shaped or finished. Different colours could also be given by immersing it in different solutions. Agates with vivid colours are treated stones: (a) lemon yellow by soaking in hydrochloric acid and heating, (b) reddish colours by simple heating or soaking in solution of ferrous nitrate and heating, (c) green by using chromium and nickel salts, (d) blue colours by soaking it in a solution of potassium ferro-cyanide and then treating it with a solution of ferric sulphate, (e) organic dyes and solutions of other coloured inorganic salts could be used. By substituting artificial colours some stones are made more attractive and costly, e.g. Swiss lapis and German lapis which are stained quartz and jasper.
2. Heat treatment.—By heat treatment some stones improve their colour, e.g. yellow topaz, which changes to an attractive pink. The stone is packed in asbestos and carefully brought to a low red heat and then cooled. Quartz, amethyst, and smoky quartz when heat treated resemble precious topaz and sell as Spanish topaz, false topaz, gold topaz, etc. The qualifying terms are even omitted and often sold as genuine topaz, but its name should be citrine.
Brownish and reddish zircons, when heated may become colourless or assume a blue colour. Colourless zircon has been long used as a gem, since it has high refractive index and strong dispersion resembling diamond to some extent. Colourless zircons are also known as Matura diamonds. Blue zircons are also popular. They are obtained in Indo-China, heat treated and cut at Bangkok, Siam. Heating it in an oxidizing or reducing atmosphere imparts different colours. The heating could also be done after cutting. The heating is done in charcoal furnaces and no chemicals
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