DIAMOND. 



The diamond, although not the most valuable of precious stones, yet unquestionably 

 exceeds all others in interest, importance and general noteworthiness. It is therefore fitting 

 that this stone should stand at the head of the series now awaiting consideration, and 

 should, moreover, receive at our hands more detailed treatment. In hardness, in the 

 perfection of its clearness and transparency, in its unique constants of optical refraction 

 and dispersion, and finally in the marvellous perfection of its lustre, the diamond surpasses 

 all other minerals. For these reasons, and despite the fact that it is not of very great 

 rarity even in faultless specimens of fair size — nine-tenths of the yearly trade in precious 

 stones being concerned with diamonds alone — it is very greatly valued as a gem ; moreover, 

 on account of its extreme hardness, it has several technical applications. 



A. CHARACTERS OF DIAMOND. 



1. CHEMICAL CHARACTERS. 



Diamond is distinguished from all other precious stones no less by its chemical 

 composition than by its unique physical characters, for no other gem consists of a single 

 element. It is pure crystallised carbon, its substance is therefore identical chemically with 

 the material of graphite and charcoal. The extraordinary difference in the appearance of 

 diamond and that of other forms of carbon depends solely on the crystallisation of the 

 material and the physical characters consequent on this. 



The fact that the one and only constituent of diamond is pure carbon was already 

 known at the end of the eighteenth century, and was suspected even earlier than this. In 

 the year 1675 Sir Isaac Newton had arrived at the conclusion that diamond must be com- 

 bustible ; this conclusion, though correct in itself, was based on theoretical grounds, now 

 known to be mistaken, connected with the high refractive index of the substance. In 

 1694!-5 researches respecting the combustibility of diamond were conducted at the 

 " Accademia del Cimento " of Florence, by the Academicians, Averani and Targioni, at the 

 instigation of the Grand Duke Cosmos III. of Tuscany. Diamonds were exposed to the 

 intense heat of a fierce charcoal fire or were placed in the focus of a large burning-glass. 

 A stone so treated did not fuse but gradually decreased in size and finally disappeared, 

 leaving behind no appreciable amount of residue. These experiments proved that the 

 substance of diamond, as such, is destroyed at a high temperature ; whether its disappear- 

 ance was due simply to volatilisation, as in the case of sal-ammoniac, was of course 

 undecided at that early date. Investigations into the chemical nature of diamond and the 



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