118 



carats, or about] i| lbs. It was bought by the South African 

 Government for ^150,000 and presented to King Edward. It 

 was of fine whiteness, but contained flaws so that it was cut up 

 into a number of gems, of which the largest weighs 516^ carats. 



Diamonds of inferior quality which, owing to bad colour or 

 imperfect crystallisation, are useless as ornamental stones serve, 

 when crushed in steel mortars, to supply the diamond powder or 

 dust used in polishing diamonds and other stones. They are 

 often called bort (also spelt boort and boart), although the strictly 

 mineralogical use of the name is limited to a special variety of 

 diamond found in nodules, rough externally, usually dark in 

 colour and showing when broken a radiated internal structure. 

 In density bort is a trifle inferior to true diamond, but it is dis- 

 tinctly harder and thus more useful as an abrading agent. In the 

 year 1845 another variety of diamond, called carbonado, was dis- 

 covered in Brazil. It occurs in small masses of dark grey or black 

 colour, appears to be an irregular aggregate of minute crystals 

 and is much harder even than bort. It has been found in Borneo, 

 but not in India, nor, as far as I know, at the Cape. Although 

 useless as an ornament its extreme hardness causes it to be highly 

 valued, and its price has risen from is. per carat to £10 or more. 

 Classing together ordinary diamond, bort and carbonado as 

 adamantine carbon we may now rapidly run over some of the 

 various uses of this form of the element. The ornamental use 

 has been sufficiently dwelt upon and so has its application in 

 cutting and polishing precious stones in general. The glazier's 

 diamond is a fragment with a sharp edge fixed in a suitable handle. 

 By its means glass plates can readily be cut into any desired sizes 

 and shapes. A deep scratch is made along the line in which the 

 glass is to be cut and this determines the direction in which it 

 will split off when a gentle force is appropriately applied. A 

 diamond drill is useful for making holes in glass and in porcelain, 

 and hence is a most valuable tool to the riveter of broken china. 

 I believe that dentists also sometimes use such drills. On a large 

 scale the diamond drill is used in rock boring. Pieces of diamond 

 or, better, carbonado, are imbedded firmly in a steel ring known 

 as the " crown." This is secured to a series of hollow tubes so 

 that the length can be increased as required. The rods are 

 rotated by steam power at the rate of about 250 revolutions per 

 minute and a pressure of perhaps half a ton or so is brought to 

 bear on the crown. Carbonado will stand a pressure of 35 tons to 

 the square inch while rotating at this high speed and hard granite 

 can be bored through at a rate of 3 inches or so per minute. 

 Water is kept circulating through the tubes to keep the crown 

 cool and wash off the debris formed. Diamonds have also been 

 used for bearings in chronometers and electric measuring instru- 

 ments of precision, also in turning, as for example in the final 

 adjustment of the axis of an astronomical transit instrument. 

 Engraving on glass can easily be done with a diamond. Thus 

 lines can be ruled on glass with a diamond point by means of a 



