88 GENERAL HARACTERS OF PRECIOUS STONES 



the stone, so far as the stone is uniformly porous and capable of absorbing liquids. This 

 process of colouring will be described in greater detail under the description of agate. The 

 exact methods of producing the artificial colours, which are black, yellow, blue, green, and 

 brown, are in many cases preserved as a trade secret. 



The writings of Pliny show that these arts were known even in ancient times, and the 

 methods then employed for colouring agates are apparently identical with those now 

 practised. At that time, however, a process was known by which certain colours, such for 

 instance as the fine green of the emerald, could be imparted to rock-crystal, a process which 

 is unknown at the present day. Rock-crystal, not being porous, cannot be coloured artificially 

 by the methods described above ; the only process known at the present day is to plunge the 

 stone when strongly heated into a cold, coloured liquid ; the abrupt change in temperature 

 causes the stone to become penetrated by numerous cracks, into which the coloured 

 liquid enters, and on evaporation deposits its colouring-matter. Rock-crystal so coloured 

 is not, however, very suitable for cutting on account of the cracks developed in it by its 

 sudden cooling ; the method therefore is of no practical significance as compared with the 

 colouring of agates, which is of great commercial importance. 



Some precious stones when subjected to the action of heat become either completely 

 decolourised or changed in colour, the original colouring-matter being in the one case 

 destroyed and in the other altered. This process, which is known as burning, is often 

 employed for increasing the natural colour of many stones, for rendering it more permanent, 

 for completely changing the colour, or for removing unsightly patches. The heating and 

 cooling must be very slow and regular and all sudden changes of temperature avoided; 

 otherwise the stone may develop cracks, or the change in the original colour may not take 

 place uniformly throughout the whole mass of the stone. For the purpose of attaining 

 a uniform rate of heating and cooling the stone is embedded in some powdered material in a 

 crucible ; the material made use of being coal-dust, fine sand, iron-filings, clay, quick- 

 lime or wood-ashes, &c. In some cases only a comparatively slight heating is necessary, 

 but often the temperature must be raised to a red-heat in order to destroy or change the 

 colour. 



The pigment to which the colour of a stone is due is in many cases unstable at high 

 temperatures, becoming changed into some other body, which is either colourless or 

 coloured differently from the original substance. Thus the change or destruction of the 

 colour of a precious stone can only be effected by burning when the pigment to which its 

 colour is due is altered on exposure to heat. 



The yellow Brazilian topaz becomes rose-red when heated, and is then known as " burnt 

 topaz." Amethyst when exposed for a short time to a gentle red-heat in a mixture of sand 

 and iron-filings loses any darkly coloured patches it may have had ; after strong and pro- 

 longed heating to redness the violet colour is changed to brownish-yellow, the stone being 

 then known as "burnt amethyst." Many of the naturally occurring brown oarnelians 

 become, when heated, a bright red ; in this case the original brown colouring-matter 

 is a hydrated oxide of iron, which is changed into the bright red anhydrous oxide by 

 the action of heat. Burning causes yellowish-red hyacinth to become colourless, and 

 at the same time appreciably increases the lustre of the stone. The blue sapphire also 

 completely loses its colour on heating. Other similar cases might be cited ; they will be 

 mentioned, since the change of colour effected by burning is of some technical importance, 

 each in its proper place with the special description of each kind of precious stone. 

 The majority of precious stones, however, undergo no alteration in colour even at the 

 highest temperatures. 



