222 DIAMONDS. 



The following table gives the specific gravities of the minerals found 

 on the sorting tables. I have also included the specific gravities of 

 two useful liquids: 



Hard graphite ~ 2.5 



Qnartzite and granite 2.6 



Beryl 2.7 



Mica 2.8 



Hornblende 3.0 



Methylene iodide 3. 3 



Diamond 3.5 



Thallium lead acetate 3.6 



Garnet 3.7 



Corundum 3.9 



Zircon 4.4 



Barytes 4. 5 



Chrome and titanic iron ore 4. 7 



Magnetite 5. 



This table shows that, if I throw the whole mixture of minerals into 

 methylene iodide, the hornblende and all above that mineral will rise 

 to the surface, while the diamond and all minerals below will sink to 

 the bottom. If I now take these heavy minerals, and throw them into 

 thallium lead acetate, they will all sink except the diamond, which 

 floats, and can be skimmed off. 



The diamond belongs to the isometric system of crystallography. It 

 frequently occurs with curved faces and edges. Twin crystals (macles) 

 are not unc; mmon. Having no double refraction, it sh( uld not act on 

 polarized light; but, as is well known, if a transparent body which 

 does not so act is submitted to strain of an irregular character, it 

 becomes doubly refracting, and in the polariscope reveals the existence 

 of the strain by brilliant colors arranged in a more or less defined pat- 

 tern, according to the state of tension in which the crystal exists. 

 Under polarized light I have examined many hundred diamond crys- 

 tals, and, with few exceptions, all sbow the presence of internal tension. 

 On rotating the polarizer, the black cross, which is most frequently 

 seen, revolves round a particular point in the inside of the crystal, and, 

 on examining this point with a high power, we see sometimes a slight 

 flaw, more rarely a minute cavity. The cavity is filled with gas at an 

 enormous pressure, and the strain is set up in the stone by the effort of 

 the gas to escape. 



It is not uncommon for a diamond to explode soon after it reaches 

 the surface, and some have been known to burst in the pockets of the 

 miners or when held in the warm hand. Large crystals are more liable 

 to burst than smaller pieces. Valuable stones have been destroyed in 

 this way, and it is whispered that cunning dealers are not averse to 

 allowing responsible clients to handle or carry in their warm pockets 

 large crystals fresh from the mine. By way of safeguard against 

 explosion, some dealers embed large diamonds in raw potato to insure 

 safe transit to England. 



