5S2 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



in Plate XIII., Figs. 4 and 4a, is known to jewellers as rose-topa?^. When of a deep red' 

 colour it is sometimes referred to as " Brazilian ruby." 



Rose-topaz, which is so rare in nature, may be produced artificially by subjecting- 

 yellow topaz, especially the Brazilian, to a gradual rise of temperature, when it assumes the 

 red colour of rose-topaz. Most of the rose-topaz sold by jewellers as " burnt " topaz is the 

 yellow variety altered by heating. The rise and fall of temperature to which the stone is 

 subjected must be very gradual, otherwise the crystal will be fissured. The darker the 

 original colour of the stone the darker will be its colour after heating. There are various- 

 methods in use for the artificial production of the red colour of rose-topaz ; in one, for 

 example, the stone is packed in a crucible with powdered charcoal, sand, ashes, or any other 

 powder, slowly heated and then slowly cooled ; in another, it is enclosed in many wrappings, 

 of tinder ; this material is then fired and the change in colour thereby effected. When the 

 latter method is adopted too great a rise of temperature must be avoided, otherwise the 

 stone will be completely decolorised, besides being rendered fissured and cloudy. 



" Burnt " topaz is much more strongly dichroic than is naturally occurring rose-topaz,., 

 or, indeed, than topaz of any other colour; the two images seen in the dichroscope,, 

 when showing the greatest possible contrast in colour, are respectively dark cherry-red 

 and honey-yellow. It has been supposed that naturally occurring rose-topaz has 

 been derived from vellow-topaz by the action of heat, but I^. von Eschwege, and others 

 familiar with the mode of occurrence of the mineral in Brazil, have shown that crystals of 

 the rare rose topaz occur together with the common yellow variety; this view, therefore, 

 cannot be correct. 



A change of colour is induced in some topazes simply by exposure to sunlight. Such. 

 a change has been observed in crystals from the Urulga river, Siberia, the original dark 

 wine-yellow colour changing after a few months' exposure to dirty white. It may be 

 mentioned here that the finest topaz crystals in the British Museum collection came from 

 this locality, and for this reason are protected from the action of light. Some pale blue 

 stones have been observed to become pale yellow after exposure to sunlight. These and 

 similar instances indicate that the colour in such cases is due to an organic substance ; those 

 stones, on the other hand, which are unaffected by light, but become red on exposure tO' 

 heat, probably owe their colour to a metallic oxide, since exposure to high temperatures, 

 would destroy any organic pigment. 



All these varieties of precious topaz are made use of as gems ; that is to say, all 

 specimens which are sufficiently transparent, finely coloured, and free from faults. Inferior- 

 stones, the so-called " fallow topaz," are crushed and powdered, and in this form utilised 

 as a hard, grinding material. The form of cutting best suited for coloured topaz is seen 

 in the faceted stones represented in Plate XIII., Figs, la, 2a, 3a, 4a ; these are step-cut,, 

 the table being somewhat small and the steps narrow and equidistant from each other. The 

 brilliant form is sometimes adopted for coloured stones, but is more often seen in colourless, 

 topaz, " pingos d'agoa," &c. Yellow topazes are not infrequently table-cut. In the case of" 

 light-coloured stones, like the Saxon topazes, for example, an added brilliancy and depth 

 of colour is given by the use of a burnished gold, or in some cases of a red, foil. Blue topaz. 

 is always backed with a pale blue shining foil ; on a dark foil it presents a peculiar and not 

 altogether attractive appearance. Only the finest and most transparent of stones, whatever- 

 be their colour, are mounted a jour. 



Topaz varies in value according to its quality ; large crystals are found quite as 

 frequently as small, consequently the value of cut stones is proportionate to their size.. 

 Topaz of any colour is not at the present time a gem favoured by the votaries of fashion,. 



