326 



SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



PHENAKITE. 



Like euclase, phenakite has but little importance as a gem. It contains beryliia but 

 no alumina, being a silicate of beryliia with the chemical formula SBeO.SiOg. It 

 crystallises in the rhombohedral system, and the crystals, which are hemihedral with parallel 

 faces, usually have the form of hexagonal prisms terminated by the faces of a rhonjbohedi"on 

 or of a hexagonal pyramid, sometimes also by small faces of other forms, as shown in 

 Fig. 65, a to c. 



Fig. 65. Crystalline forms of phenakite. 



The cleavage of phenakite is very imperfect and its fracture is conchoidal. Its hardness 

 (H = 7-J — 8) is slightly greater than that of either beryl or euclase ; while its specific 

 gravity (sp. gr. = 2-95 to 3-0) is rather less than that of euclase, and the mineral just 

 Hoats in liquid No. 3. 



The lustre of a fractured surface of phenakite is brilhant and vitreous ; that of the 

 natural crystal faces is usually, however, much duller. It is susceptible of a very briUiant 

 polish, and has then a lustre comparable to that of the sapphire. The mineral is frequently 

 water-clear, but may be cloudy or only translucent. Usually it is colourless, but yellow, 

 brown, and rose-red phenakite have been found. Except for its brilliant lustre, colourless, 

 water-clear phenakite resembles rock-crystal in appearance; the refraction and double 

 refraction of the two stones are very much the same, the former being a little greater and 

 the latter a little less in phenakite. Water-clear phenakites, and indeed all stones of this 

 description, are cut in the brilliant form, a form which displays the lustre and brilliancy of 

 the stone to the best advantage. A phenakite brilliant has certain resemblances to the 

 diamond, but never shows the brilliant play of prismatic colours characteristic of this gem. 

 Two very fine faceted phenakites, weighing 43 and 34 carats respectively, are exhibited in 

 the British Museum of Natural History. 



Phenakite is a less rare mineral than is euclase, but the number of localities at which it 

 is found is almost as limited as for euclase. The white phenakite, which occurs with emerald 

 and alexandrite embedded in mica-schist at Takovaya in the Ekaterinburg district of the Urals 

 was the first to be met with. At this locality crystals with a thickness of 10 centimetres 

 and a weight of IJ pounds are found. Phenakite also occurs with topaz and green felspar 

 (amazon-stone) at Miask, on Lake Ilmen, in the Urals, but this locality is of less importance. 

 The Takovaya stones, those at least which are sufficiently transparent, are usually cut in 

 Ekaterinburg, and are placed on the market at the fairs of Nizhniy Novgorod. Not many 



