96 GENERAL CHARACTERS OF PRECIOUS STONES 



tion of the stone according to the methods of scientific mineralogy. These methods will be 

 detailed in the third section of this book, and under the description of each kind of 

 precious stone it will be stated how that stone may be distinguished from others which 

 resemble it in general appearance. 



2. Doublets. — A cut stone, which consists of an upper and a lower portion cemented 

 together so as to present the appearance of a single stone, is known as a doublet. There is 

 less deception here when the two portions consist of genuine material, for example diamond, 

 the two small stones forming together a large and apparently single stone, which, if really 

 single, would be of greater value than the two stones mounted separately. Such a 

 combination of two small, genuine stones may be referred to as a genuine doublet. 



Very frequently, however, the upper portion only of the doublet is genuine, the lower 

 portion being cut from comparatively worthless material, such as quartz or glass. When 

 skilfully contrived, such a doublet has the appearance of a single stone of the same 

 material as that of the upper portion, and at a first glance shows all the beauty such a stone 

 would have. Such combinations are known, from the material of their upper portion, as 

 diamond-, ruby-, sapphire-doublets, &c. It is said that in Antwerp at the present time 

 diamonds and colourless sapphires are often combined in this way. The advantage to be 

 derived from such a proceeding is very evident ; the dealer is able to sell what appears to be 

 a large stone but which in reality consists of comparatively little genuine material. When 

 the lower half of a semi-genuine doublet of this kind consists of glass, this may be fused to 

 the upper portion and a more intimate and permanent union effected than if the two 

 portions were fixed together by cement or mastic. 



A fi-aud of this kind is difficult to detect when the stone is mounted in a closed setting. 

 Detection is easier when the stone is unmounted, for with the help of a good lens the line of 

 junction of the two portions may be seen, or sometimes the prismatic colours of thin films 

 may be visible where air has penetrated along the plane of junction. The two portions of a 

 doublet cemented together by mastic will fall apart when the stone is placed in hot water, 

 but this of course will not happen when the two parts are fused together. The compound 

 nature of a doublet made, of glass, and of a doubly refracting stone such as ruby, is easilv 

 recognised by the different behaviour of the two parts in polarised light. The difference in 

 the refractive index of the two parts will also serve the same purpose, especially with doublets 

 of colourless stones such as diamond and rock-crystal. For the purpose of demonstrating 

 this difference the stone is immersed in a strongly refracting liquid, such as methylene 

 iodide, and this diluted until one or other part of the stone becomes invisible. As explained 

 in the section on optics, this will happen when the index of refraction of the liquid is the 

 same as that of the stone. In the case of a doublet of quartz and diamond immersed in 

 methylene iodide diluted with benzene the quartz will become invisible, while the more 

 strongly refracting diamond will still preserve its sharp outlines. 



Such elaborate devices for the detection of a doublet are, however, only necessary when 

 it is very cleverly made, and when the substances of the two portions are well matched. 

 Indian jewellers are specially expert in the production of good doublets. When less 

 carefully contrived and put together, there will be sufficient contrast between the two 

 portions to make the doublet easily recognisable as such. 



Doublets, of which the upper portion consists of rock-crystal or other colourless stone, 

 and the lower part of a coloured glass, are known fis false doublets. Here the colour of the 

 lower portion is imparted to the upper harder portion. The same effect is obtained when 

 between the upper and lower uncoloured portions is placed a thin layer of coloured material, 

 a plate of metal, or even a piece of coloured gelatine-paper. When the two portions are 



