GLASS IMITATIONS 97 



differently coloured, the stone may be instantly recognised as a doublet by holding it up to 

 the light and viewing it from the side, or observation with a lens will disclose the coloured 

 layer between the two colourless portions. Here, as in all cases, when examining a doublet, 

 the stone must of course be unmounted. 



The construction of hollow doublets is somewhat peculiar ; these consist of an upper 

 portion of rock-crystal or colourless glass, which is hollowed out below, the walls of the 

 hollow being finely polished. The cavity so hollowed out is filled with a coloured liquid, 

 and closed with a plate of rock-crystal or glass, or by a complete lower portion of the same 

 material. The whole doublet if viewed from above, that is from the table, appears of the 

 same colour as the liquid ; when viewed from the side, however, the boundary of the cavity 

 containing the coloured liquid is plainly visible. 



3. Glass imitations. — The manufacture of glass imitations of precious stones has 

 reached a high degree of perfection. The varieties of glass suitable for this purpose are 

 known as paste, and this name is also applied to the imitations themselves, which are often 

 substituted for genuine precious stones. This species of fraud, which is common enough at 

 the present day, was known and practised by the ancients, and attention was drawn to it by 

 Pliny, who gave eloquent warnings on the subject. 



The manufacturer of such kinds of glass aims at producing a substance which will, as far 

 as possible, exhibit the more beautiful and valuable characters of genuine precious stones, and 

 which, at the same time, will be in price as far removed as possible from the latter. The 

 method at present followed is to produce a mass of glass which shall be as clear, transparent, 

 and colourless as possible. When a material for coloured imitations is required, the 

 colourless glass must be again fused with some metallic oxide capable of producing the 

 desired colour. 



The majority of precious stones can be so successfully imitated in glass that only a 

 very practised eye can distinguish without more detailed examination a genuine stone from 

 its paste imitation. Some of these artificial glasses possess not only the same clearness 

 and transparency, but also in a large measure the lustre and high index of refraction 

 and dispersion of a diamond of the purest water. Others again may exhibit a colour 

 comparable with that of the finest ruby, sapphire, emerald, or topaz, &c. 



The one point, however, in which all artificial imitations fail is hardness ; they have the 

 hardness of glass (H = 5), and are, as a rule, softer than ordinary window-glass. In spite of 

 this fact they take a high polish, which, however, after use is soon lost ; neither is the 

 sharpness of their corners and edges retained for any length of time. Although when new 

 these glass imitations are very similar in general appearance to genuine stones, and may be 

 substituted for them with, in most cases, but little fear of detection, yet after they have been 

 in use a short time they become dull and anything but beautiful objects. If it were possible 

 to give these artificial glasses the hardness of true precious stones, they would in many cases 

 be almost as suitable for personal ornament as are the latter, since the objection to their 

 use, which has been j ust mentioned, would not exist. This drawback to the utilisation of 

 the soft artificial glasses as precious stones is frequently overcome by the use of the semi- 

 genuine doublet, the lower and larger portion of the doublet being of glass, and the upper 

 smaller portion of some hard stone, such as quartz. 



An artificial glass may, in almost all cases, be distinguished from a genuine stone by its 

 lack of hardness. Glass is, as we have already seen, easily scratched by a hard steel point, 

 which will not touch the great majority of precious stones. In addition to other methods, 

 an aluminium pencil has recently been used for the purpose of distinguishing between genuine 

 stones and glass imitations ; the point of the pencil when drawn over glass leaves a shining, 



