LUSTRE 36 



cloudy and opaque as to be useless as a gem. The cloudiness in such a case is due to the 

 presence of numerous cracks and fissures in the stone, or to foreign matter included in its 

 substance, either of which obstructs the free passage of light through the stone, scattering it 

 at the surface. 



Cracks and fissures are specially frequent in stones, such as topaz, which possess a good 

 cleavage. They are not confined to such stones however, but are frequently found in those 

 which possess no distinct cleavage, su(;h, for instance, as the green emerald, which is ahnost 

 always penetrated by numerous cracks. Their presence naturally reduces the transparency 

 of the stone ; perfectly faultless specimens of emerald are of the greatest rarity. Enclosures 

 of foreign matter are of not uncommon occurrence in crystallised minerals. Thus black and 

 other coloured grains are sometimes found in the substance of diamond, and numerous 

 scales of mica in that of emerald. These enclosures may be so small as to be only visible 

 under a high power of the microscope ; when distributed evenly throughout the substance 

 of the stone, as they usually are, their effect is to make the whole stone cloudy. On the 

 other hand a few enclosures of larger size will leave portions of the stone clear. 



The substance of some precious stones contains enormous numbers of extremely minute 

 cavities often arranged in strings, and giving rise to a silky or cloudy glimmering or sheen, 

 which greatly impairs the transparency and beauty of the stone in which they occur. This 

 kind of cloudiness, present as a fault in precious stones which would otherwise be trans- 

 parent, is known to jewellers as " silk." 



The transparency of a mineral is largely dependent upon its structure. While crystals,, 

 at least as far as precious stones are concerned, are usually ti'ansparent, an aggregate of smalt 

 crystals of the same kind, that is a compact crystalline aggregate, is usually opaque, or at, 

 most translucent. The reason for this is clear ; at the boundaries of each of the minute- 

 constituent grains, fibres or scales of a crystalline aggregate, a certain amount of light willl 

 be scattered and lost, and thus never reach the eye. For this reason chalcedony, chrysoprasey 

 &c., are not transparent, although they are built up of minute transparent crystalline grains 

 of quartz, a mineral which in its most perfectly crystallised and transparent condition is 

 known as rock-crystal. 



The varying degrees of transparency possessed by diff^erent precious stones for Rontgen 

 (X) I'ays has an important application in their determination ; this will be dealt with in 

 the third part of the present work. 



2. LUSTEE. 



When light falls upon a body a portion of it is thrown back or reflected from the 

 surface, while another portion enters its substance. It is the portion of light reflected from 

 the surface on which depends the lustre of the body. 



The lustre of a body varies with the proportion of light reflected at its surface; 

 hence different degrees of intensity of lustre, distinguished as splendent, shining, 

 glistening, glimmering, and dull, exist in different stones. The lustre of a perfectly smooth 

 surface, reflecting a sharp image of an object, is splendent ; a surface which reflects a less 

 sharp image is shining ; the image reflected from a surface of glistening lustre is still less 

 sharp ; a surface giving only a feeble reflection has glimmering lustre ; lastly, a dull surface 

 reflects no light. 



The lustre of most precious stones, especially the more valuable, is splendent, as is 

 often seen on the natural faces of a crystal, but more frequently when the stone has been cut 

 and polished. A high degree of lustre adds very considerably to the beauty of a stone, and 

 the object of polishing is to render this quality as perfect as possible. What is known as 



