330 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



specially noticeable on the prism faces. The lustre of a cut topaz which has been well 

 polished is almost comparable with that of the diamond. 



Topaz is not a highly refractive substance, its indices of refraction only slightly 

 exceeding 1*6. Being a rhombic mineral it is doubly refracting, but here again the two 

 indices of refraction differ from each other only slightly, so that the double refraction is not 

 strong. The refractive indices for variously coloured light also differ but slightly, so that 

 the dispersion is likewise small. Very little play of prismatic colours is shown, therefore, by 

 & faceted topaz, which in other respects somewhat resembles the diamond. The gi'eatest, 

 mean, and least value of the refractive index of one and the same crystal for red and for 

 violet light is given in the table below : 



Ked light . . . 1-618 1-610 1-608 



Violet light . . . 1'635 1-627 1-625 



These values will differ slightly for othef crystals, especially when the latter differ in colour 

 or in place of origin, owing to variations in chemical composition. 



The range of colour exhibited by topaz is considerable. The purest variety is perfectly 

 colourless and pellucid, and is of frequent occurrence. It is found as crystals at Miask, in 

 the Urals, among other places, and in still greater abundance, in the form of rounded, 

 water-worn pebbles, in the streams and rivers of Diamantina and specially of Minas Novas, 

 in the State of Minas Geraes, Brazil. These pebbles, which are met with also in Australia, 

 especially in New South Wales, are often perfectly colourless and transparent, and are then 

 known to the Brazilians as "pingos d'agoa " (drops of water). Colourless topaz is sometimes 

 known to the trade as "goutte d'eau,'''' the French equivalent of the term. 



The " Braganza," a supposed diamond belonging to the Portuguese crown jewels and 

 weighing 1680 carats, is probably nothing more than one of these topaz pebbles of unusual 

 transparency and beauty. These stones are often called " slave's diamonds " on account of 

 their resemblance to the diamond. When cut they are not infrequently passed off' as 

 ■diamonds, and as the specific gravity is almost exactly the same as that of diamond, resort 

 to some other means by which they may be distinguished is necessary. This is a matter of 

 no great difficulty, for topaz, beside being much less hard than diamond, is also doubly 

 refracting, while diamond, like other cubic minerals, is singly refracting. Topaz may, 

 however, be distinguished from other colourless stones, such as rock-crystal, phenakite, and 

 ■colourless sapphire, by the difference in specific gravity. Thus, in methylene iodide 

 rock-crystal (sp. gr. = 2-65) and phenakite (sp. gr. = 2-98-3-0) both float, while topaz 

 quickly sinks. In the heaviest liquid, on the other hand, colourless sapphire sinks while 

 topaz floats. Moreover, topaz differs from the minerals j ust mentioned in its capacity for 

 acquiring charges of electricity. 



The colour of topaz, though usually pale, is sometimes deep and intense, and in this 

 ■case the dichroism of the mineral, though scarcely apparent to the naked eye, can be easily 

 observed in the dichroscope. 



Topaz is very frequently blue in colour, either a pure blue or a blue tinged with 

 green, but scarcely ever a pure green. A blue crystal of topaz is represented in Plate XIII., 

 Fig. 1, and the gem cut from it in Fig. la of the same plate. Dark blue topaz is very 

 unusual; the mineral is almost invariably pale in shade, sometimes so pale that it may be 

 more correctly described as blue-white. Such stones are common among the "pingos 

 d'agoa'" of Brazil as well as amongst crystals from Mursinka, near Ekaterinburg, in the 

 Urals ; these latter are known as Siberim or Tauridan topaz. Stones of a darker shade of 

 pale blue are referred to as " Brazilian mpphire,'" a term which is applied also to the blue 



