GEMS, ARTIFICIAL 631) 



The colour of the true emerald is a very bright green, the pale green and bluish green 

 varieties being distinguished as beryl or aqua-marine : tha emerald is rarely without 

 flaws, which are so characteristic as to be imitated in mock stones. The chrysolite or 

 peridot presents a very dull green colour, and is so soft as to bo easily abraded when 

 worn, and thus soon loses its brilliancy of surface and sharpness of edge. The dichroi- 

 scope resolves the colour of the emerald into a yellowish green and a bluish green ; 

 of the tourmaline into a pale green and a dark blackish green ; and of the Oriental 

 emerald (green corundum) into green and yellowish green. But the Oriental emerald 

 rarely occurs in trade, and is said indeed to be the rarest of all stones. 



Slue Gems. 



Sapphire. . . . . S.G. 4 H. 9 D.R. 



Blue topaz . . . 3-5 ,,8 D.R. 



Blue beryl ,27 ,,8 D.K. 



Blue tourmaline . . , 3 7'5 D.R. 



By means of the dichroiscope, the colour of sapphire is resolved into a pale and a dark 

 blue ; that of topaz into a whitish blue, and a greenish blue ; of beryl into similar 

 tints to those of topaz ; and of tourmaline into a pale bluish green and a very dark, 

 bluish green, Cyanite is occasionally cut as a jeweller's stone. 



Purple or Violet Gems. 



Amethyst . . V *' . S.G. 2'6 H. 7'5 D.K. 

 Oriental amethyst . . . . 4 ,,9 D.K. 



lolite . . . . ; '".' '-.'. 2-6 7 D.R. 



The common amethyst (purple quartz) and the Oriental amethyst (purple corundum) 

 are very similar in colour, but are readily distinguished by specific gravity and hard- 

 ness ;. it should, however, be remarked that jewellers are in the habit of calling any 

 stone of fine colour ' Oriental amethyst,' though it may have no mineralogical claim 

 to the name. lolite (dichroite) is characterized by its strongly-marked dichroism, 

 commonly exhibiting a blue colour in one direction and a brownish yellow in another; 

 some varieties are pleochroic, or exhibit several distinct shades in different directions. 



Imitation Gems. In determining a doubtful specimen the observer should always 

 bear in mind the possibility of its being merely a paste, or imitation in fine glass. 

 The inferior hardness of glass, compared with that of any precious stone, renders the 

 determination easy. Although the hardness of glass varies with its composition, it 

 rarely exceeds 5'5 ; all glass may, therefore, be readily scratched with a steel file. The 

 more brilliant the glass the softer it generally becomes ; and, therefore, pastes when 

 worn soon get blunt and jagged at their edges and corners, whilst the surfaces become 

 dull and scratched : such imperfections, if not readily seen by the naked eye, are at 

 once detected by aid of a lens. The specific gravity of. paste is not an available test, 

 for it varies from 2'5 to 4'5 according to its composition ; the greater the proportion 

 of lead present, the more brilliant, the denser, the softer, and the more fusible is the 

 glass. Like all uncrystallized substances, glass is singly-refracting. 



In examining a stone, it is also necessary to guard against doublets or semi-stones. 

 These are made up of two pieces ; the upper half being a real stone, and therefore 

 presenting its proper degree of hardness, whilst the Jower, half . is either an inferior 

 stone or mere glass. When such stones are set, it is difficult to detect the fraud. The 

 deception arising from the use of foil has already been noticed in this article. F.W.R. 



GEMS, ARTIFICIAL. These are glasses, the material of which they are com- 

 posed being called Strass. 



Strata, the paste or glass which generally forms the principal ingredient of imitation 

 gems, is called after the name of a German jeweller by whom it was invented, at the 

 commencement of the last century. It is composed of silica, potash, borax, the various 

 oxides of lead, and sometimes of arsenic : chemically it may be regarded as a double 

 silicate of potash and lead. 



The silica may be furnished either by rock crystal, white sand, or flint ; but, of these, 

 the first is to be preferred, one of the principal considerations in these preparations 

 being the extreme purity of the materials or ingredients employed. In this manu- 

 facture, which is of more importance, and attended with greater difficulty than most 

 persons imagine, perfect success (independently of the choice of materials) depends 

 upon the care taken, and the precautions to be observed. No crucibles should be 

 used but those which have been proved, both as regards their composition, their 

 power of withstanding the strongest heat, and their impenetrability to the action of 

 metallic oxides. 



All the substances to be melted should be first pulverised, and even ground with 



