TOPAZ. 



Remarkable clearness and transpa- 

 rency, capacity of taking a high polish 

 and hardness and weight greater than 

 that of quartz. These are the qualities 

 in which Topaz excels as a gem. Nu- 

 merous other stones of inferior quality 

 masquerade under its name, however, 

 and this fact may account for the de- 

 cline in popularity which the stone has 

 suffered in recent years. True Topaz is 

 a silicate of alumina, containing hydroxyl 

 and fluorine. Its hardness is 8 in the 

 scale in which quartz is 7. Hence it will 

 scratch the latter mineral and may thus 

 be distinguished from it. It is also re- 

 markably heavy, considering its compo- 

 sition, it being three and one-half times 

 as heavy as water, while quartz is only 

 two and one-half times as heavy. Owing 

 to this unusual specific gravity, experts 

 accustomed to handling gems can fre- 

 quently pick out the Topaz from a miscel- 

 laneous lot of precious stones without re- 

 moving their wrappings. 



The color typically associated with To- 

 paz in its use as a gem is yellow. Yet 

 the mineral species exhibits many other 

 shades of color, which, when present in 

 crystals of sufficient clearness and purity, 

 answer equally well for gem purposes. 

 These other shades, most of which are 

 represented in the accompanying plate, 

 are grayish, greenish, bluish and reddish. 

 Topaz may also be quite colorless. The 

 yellow color of the Brazilian Topaz can 

 be changed by heating to a pale rose pink 

 and the gem is often treated in this way. 

 The degree of heat employed is not high, 

 and both heating and cooling must be 

 performed gradually. Warming in a 

 sand bath at a low red heat is the method 

 usually employed, or the stone may be 

 wrapped in German tinder and the latter 

 set on fire. Only stones of a brown yel- 

 low color yield the pink ; the pale yellow 

 stones turn white when so treated. Once 

 the pink color is obtained it is perma- 

 nent. The natural colors of Topaz are 

 in general perfectly durable, although 



some of the deep wine yellow Topazes 

 from Russia fade on exposure to day- 

 light. 



Topaz is infusible before the blow- 

 pipe. It is not affected by hydrochloric 

 acid, but is partially decomposed by sul- 

 phuric acid and then yields hydrofluoric 

 acid. If the latter experiment is tried 

 in a closed glass tube the formation of 

 the hydrofluoric acid is made evident by 

 the etching and clouding of the walls of 

 the tube. The powdered stone should 

 be mixed with acid sulphate of potash 

 for this experiment. The powdered min- 

 eral, when heated with cobalt nitrate, 

 assumes a fine blue color, due to the 

 alumina which it contains. One of the 

 most convenient means of distinguishing 

 Topaz from other stones used to imitate 

 it is through its property of becoming 

 electric by heat, friction or pressure. 

 This electrical condition is evidenced, as 

 in the case of tourmaline, by the power 

 the stone acquires to pick up and hold 

 bits of tissue paper, straws, etc. Some- 

 times the friction from merely rubbing 

 the stone between the fingers will be suf- 

 ficient to produce this electrical condi- 

 tion, while many Brazilian Topazes, if 

 simply pressed between the fingers, es- 

 pecially in the direction of the prismatic 

 axis, become electric. The electrical 

 condition often persists from twenty-four 

 to thirty hours. 



The crystals of Topaz belong to the 

 orthorhombic system of crystallization. 

 They are usually elongated in the direc- 

 tion of the prism and have sharp, bright 

 faces. They vary much in size and of- 

 ten are large. One crystal weighing 

 twenty-five pounds was found in Siberia. 



A well-marked characteristic of all To- 

 paz crystals is their tendency to cleave 

 across the prism parallel with its base. 

 Such a cleavage plane can be seen cut- 

 ting across the crystal shown in the up- 

 per right-hand corner of the accompany- 

 ing plate. This cleavage is so marked 

 and the cleavage plane so bright and flat 



216 



