308 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



white colour. Only the transparent or semi-transparent precious beryl is used for cutting 

 as gems ; this is usually of a beautiful colour, often green or blue, but sometimes yellow. 

 All varieties of beryl have the common vitreous lustre. 



In correspondence with its crystalline form, all hexagonal crystals being birefringent, 

 beryl is doubly refracting, only, however, to a small extent, for the greater and lesser 

 indices of refraction for the same colour differ but slightly. Its refraction is also small. In 

 the case of emerald from Muzo the greater and lesser refractive indices have been determined 

 to be 1-584 and 1-578, and in the case of Siberian aquamarine 1582 and 1-576. The 

 dispersion is also small, the refractive coefficients given by the same crystal for differently 

 coloured rays of light differing but slightly. This may be seen by comparing the refractive 

 indices of a crystal of beryl for red, yellow, and green light which are given below : 



Greater refractive index 



Lesser „ „ ... 



It follows, then, that scarcely any play of prismatic colours, such as is characteristic of 

 the diamond, is seen in beryl ; its beauty depends mainly on its strong lustre, and on its fine 

 body-colour. There is a certain amount of variety in this latter character, but very much 

 less than in corundum. Green and bluish-green beryl is most common, yellow rather less 

 so, pale red and water-clear stones rare. Different varieties of beryl are distinguished by 

 their colour ; bright grass-green beryl being known as emerald. Other varieties, which are 

 always of a pale colour, are referred to as precious or nohle beryl. Of these light-coloured 

 varieties, the pale blue, bluish-green, or yellowish-blue, is distinguished as aquamarine, the 

 yellowish-green as " aquamanne-chrysolite,'" while the yellow variety is known to jewellers as 

 heryl, and when of a pure golden-yellow as golden beryl. Of all these varieties the emerald 

 is by far the most important as a precious stone, ranking, indeed, with the costliest of 

 gems ; aquamarine is also much used, while the other varieties are of less importance. 



All transparent beryls, whatever their colour if not too pale, are distinctly dichroic ; 

 differences in colour can often be observed with the naked eye, and with the dichroscope 

 are, as a rule, unmistakable. On account of this property, it is possible, therefore, to 

 distinguish genuine beryl from coloured glass imitations, or from other gem-stones which it 

 may resemble in appearance. 



The characters by which the different varieties of beryl are distinguished must now be 

 considered. These varieties, which differ from each other principally in colour, are by no 

 means equally valuable as gems, the emerald being by far the most costly. 



EMERALD. 



Emerald is the name given to beryl of a pure and intense green colour ; the particular 

 shade of colour seen in this variety of beryl is often alluded to in ordinary language as 

 emerald-green, but emeralds may be also grass-green, green tinged with yellow, or celadon- 

 green tinged with grey. Beryls of a bluish-green colour are not included in this variety. 

 Many specimens of emerald are very pale in colour, varying in intensity down to greenish- 

 white ; these are, however, not cut as gems ; only those of a beautiful and deep emerald-green 

 to grass-green are highly prized. The particular shade of green characteristic of the 

 emerald is shown in Plate XII., Figs. 1 — 3 ; it is almost unrivalled in depth and brilliancy, 

 and is often compared to the fresh green of a meadow in spring. The finest stones possess 

 a peculiar velvety lustre, like that shown by some dark blue sapphires. 



According to F. Wcihler, the colour of emerald from Muzo in Colombia withstood 



