366 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



These darker and more dense tourmalines are not cut, the paler and lighter varieties only 

 being suitable for gems ; the specific gravity of the latter lies between 3-0 and a value slightly 

 in excess of 3'1. 



The common vitreous lustre of tourmaline is much heightened by polishing ; indeed, all 

 tourmalines are susceptible of a good polish. In respect of brilliancy of lustre tourmaline 

 surpasses beryl, especially by artificial light; the latter mineral, however, is superior to 

 tourmaline in the richness of its colours. 



Tourmaline varies in transparency and colour to a considerable extent. Most of it is 

 black, or, at least, vory deeply coloured, and quite opaque in mass, but in sufficiently thin 

 splinters it is transparent. Tourmaline which is transparent in mass is comparatively rare, 

 and is distinguished as " precious tourmaline " in contrast to the darkly coloured and opaque 

 "common tourmaline." Precious tourmaline is much lighter in colour, and is the only 

 variety suitable for cutting as a gem ; perfectly clear and faultless specimens are rare and 

 command a moderately high price. Tourmaline is very variable in colour, more so than 

 most other minerals used as gems ; the colour variations will be dealt with in some detail 

 below. I 



In striking contrast to black tourmaline, the so-called "schorl," is the colourless 

 variety, which, however, even when transparent, is not perfectly water-clear, being usually 

 faintly tinged with red or green. Colourless tourmaline is known to mineralogists as 

 achroite ; it is rarely cut as a precious stone and is not of frequent occurrence. Red 

 tourmaline is of greater importance: this variety is of a pale rose-red shade, lighter in 

 some specimens, darker in others. It may even be of a fine ruby-red colour, and is then 

 known as rubellite or, on account of its occurrence in Siberia, as siberite. A violet tint 

 is sometimes seen owing to the admixture of blue with the typical red colour. The darker 

 red tourmaline only is important as a gem, rose-red specimens not being used at all. Precious 

 tourmaline of various tints of green, ranging in shade from pale to dark, is of much more 

 frequent occurrence. A pure emerald-green is rai-ely seen, bluish-green and especially 

 yellowish-green being much more common. Blue tourmaline, at least in transparent 

 specimens, is rare ; it is known to mineralogists as indicolite, and is usually somewhat deeply 

 coloured, being sometimes a pure indigo-blue, at other times showing a pronounced tinge of 

 green. Brown tourmaline, to which the name dravite is applied, is also widely distributed ; 

 it is either pure brown, greenish-bro\\n, or reddish-brown, and ranges in shade from a colour 

 of considerable depth to somewhat pale brownish-yellow and straw-yellow. 



Ci'ystals of tourmaline may be of one uniform colour throughout or differently coloured 

 in different portions. Thus, for example, the terminal portions of prisms of colourless 

 tourmaline from the island of Elba are frequently black ; such crystals are known as " negro- 

 heads.'' The change from colourless to black material is abrupt, but there is no absolutely 

 sharp boundary ; these conditions are sometimes reversed, a black crystal having white ends. 

 Prismatic crystals coloured rose-red at one end and green at the other, as represented in 

 Plate XV., Fig. 5, are not rare; the transition from one colour to another in the middle of 

 the ci'ystal is gradual. Also of interest are the ci'ystals from Chesterfield, in Massachusetts, 

 and from other localities, in which there is a red central portion enclosed by a green shell, 

 the transition between the differently coloured portions being sharp, as represented in 

 Plate XV., Figs. 8 and 9. 



The colour of tourmaline is not due to the mechanical intermixture of pigment, but 

 is a property of the substance of the mineral itself, and even in the darkest crystals is 

 distributed with perfect regularity. It may be supposed that the fundamental molecules 

 mentioned above have each their own characteristic colour, and that the different colours 



