382 TOPAZ. 



Comp. Silicate of alumina with fluoride 

 of silicium, or fluoride of aluminium ; or GAP 

 Si2 + (3Air5 + 2SiF3). 



ff\ 



Fig. 436. 



Fig. 437. 



Analyses, bj'- Forchammer : (a) Saxon: 

 {h) Brazilian. 



(«) (h) 



Silica . . . 22-3 25-1 



Alumina . . . 54-3 53-8 



Silicium . . .6*5 5*8 



Fluor . . . 17-3 15-7 



100-4 100-4 



A sensible quantity of Vanadium has 

 been found by Mons. Henri Sainte-Claire 

 Deville in Topaz from Brazil. 



Mons. Deville, also, states . that Topaz 

 has been formed in the wet way (as was 

 proved by Brewster's observations on the 

 liquids it sometimes contains), probably 

 from hydro-fiuo-aluminous acids. 



BB alone on charcoal infusible ; exposed 

 to a strong heat blisters rise on the sui-face, 

 which burst as soon as formed, evolving 

 hydrofluoric acid, or probably fluoride of 

 silicium. Fuses slowly to a transparent 

 glass with borax. 



Not acted on by muriatic acid. Digested 

 for some time in sulphuric acid, yields hydro- 

 fluoric acid. 



The Topaz is divided by jewellers into 

 two kinds. Oriental and Occidental. The 

 first of these is, in fact, not Topaz, but a 

 hyalin Corundum, which has been described 

 already in its proper place. (See Oriental 

 Topaz.) Occidental Topaz may be divided 

 into three varieties : viz. yellow, blue, and 

 •white. 



I. Yellow Topaz. The colour is gene- 

 rally a beautiful wine-yellow of diff"erent 

 degrees of intensity, but at the same 

 time very limpid. The stone is valued 

 in proportion to the fulness of its co- 

 lour, provided it loses no portion of its 

 brightness. Yellow Topaz occurs of large 

 size compared with many other precious 

 stones, and is universally esteemed on ac- 

 count of the rich warm tone of its colour, 

 which it retains even by the side of the 

 Diamond. 



Considerable skill and taste are required 



TOPAZ. 



in cutting this stone. To displa}' it to the 

 greatest advantage, the table should be 

 perfectly sj-mmetrical, and not too large, 

 the bizel should be of sufficient breadth, and 

 the under side should be formed into delicate 

 steps, not into pavilion facets. One of the 

 most remarkable properties of the Brazilian 

 Topaz is that of changing from yellow to 

 pink or pale crimson on exposure to a 

 gentle heat and of retaining this colour 

 permanently. This process was discovered 

 by Dunielle, a Parisian jeweller, in 1750. 

 He used to heat Brazilian Topaz in a sand 

 bath, but the process is much simplified 

 now, and, consists in closely wrapping the 

 stone to be operated on in amadou, and 

 binding it round with a piece of tin wire. 

 The amadou is lighted, and when con- 

 sumed, the Topaz is found to be rose- 

 coloured, without producing any injurj^ to 

 the polish of the stone, which has only to 

 be cleaned to be restored to all its former 

 brilliancy. The deeper the tint of the 

 original colour, the deeper will be the rose 

 colour — which sometimes becomes wine- 

 coloured like that of the Balas Ruby, with 

 which it is often confounded. 



Many stones called Brazilian Rubies are 

 only Topaz which have been successfully 

 operated upon in this manner. It is, how- 

 ever, rather a hazardous experiment to 

 perform, the Topaz being very apt to crack 

 and flaw by the action of fire. The finest 

 Brazilian j'ellow Topazes come from Villa 

 Rica, where they are found in the form of 

 loose crystals or rounded pebbles, or in. 

 veins or nests imbedded in Lithomarge. 



The Saxon Topaz is generally of a pale 

 yellow bordering on canary colour. It oc- 

 curs in quadrangular prisms, terminated by 

 a truncated pyramid with unequal faces. 

 Sometimes (but rarely) it possesses bril- 

 liancy, but, unless of extraordinary size and 

 beauty, this variety is of scarcely any value. 

 The colour of this variety disappears in 

 proportion as it is heated, and returns as 

 gradually on cooling. It is found in the rock 

 of Schneckenstein, in the valley of Damberg. 



Mexican Topaz is nearly similar to the 

 last in qualities and defects, only that it is 

 more variable in its diff'erent tints. 



That from Siberia is verj- limpid, of a 

 beautiful jonquil-yellow when fine, but it 

 more frequently resembles Aquamarine than 

 Topaz, at least in colour. It is brought from 

 the Uralian and Altai mountains ; Miask, iu | 

 Siberia ; and from Kamschatka, of green and ! 

 blue colours. \ 



II. The Blue or Brazilian Sapphire, as it 

 has been called hy some authors, varies isi i 



