136 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



rays may have a regular star-like arrangement, or may form a figure like the club of playing 

 cards, on the faces of the octahedron. 



Of interest is the fact that diamond sometimes shows a play of colours like that of 

 precious opal. Des Cloizeaux mentioned a few such stones which differed from opal in this 

 respect only in that the colours were less brilliant. Pale blue and yellow stones have been 

 reported by Mawe to show a somewhat similar appearance. 



The colour of diamonds is by no means in every case unchanging and unalterable. 

 Some stones are bleached by sunlight; thus a red diamond on exposure to sunlight is 

 reported to have gradually lost its colour and become white. A diamond in the possession 

 of the Parisian jeweller, Halphen, undergoes a peculiar change in colour on exposure to heat. 

 This stone, which is of a faint brownish colour and weighs four grams (about twenty carats), 

 assumes in the fire a beautiful rose-red colour. If kept in darkness this colour is retained 

 for about ten days, after which it returns to its original brownish colour ; should the stone 

 be exposed to diffuse daylight, or to the direct rays of the sun, the change to the original 

 colour is much more rapid. The change to rose-red can be produced at will by again 

 exposing the stone to the action of heat. Could means be devised for retaining the rose-red 

 colour of the stone, the possessor would benefit to the extent of many thousand francs, since 

 it is valued when brown at 60,000 francs, and when rose-red at from 150,000 to 200,000 

 francs. Halphen has also seen a diamond which when rubbed assumed a rose-red colour ; 

 this colour, however, was lost again almost immediately. 



The colour of diamonds is in some cases affected by exposure to a high temperature. 

 According to Des Cloizeaux, pale green diamonds, after being heated in the oxyhydrogen 

 flame, became light yellow ; brown crystals under the same conditions become greyish. 

 Baumhauer also witnessed the colour of one diamond change from green to yellow, and of 

 another from dark green to violet, under the influence of a high temperature. Wohler caused 

 green diamonds to assume a brown colour by heating them, but found that brown stones 

 remained unaltered in colour. Yellow diamonds, especially those from the Cape, retain their 

 colour at the highest temperatures. 



It has already been mentioned that very faintly coloured stones command a somewhat 

 lower price than do those which are perfectly colourless. Hence many attempts have been 

 made to transform faintly coloured stones into the more valuable colourless stones. This 

 is readily effected in those Brazilian diamonds, which have a colourless central portion 

 sun'ounded by a coloured external layer of no great thickness. The outer coloured layer 

 is in these cases simply burnt away by heating the stone in a crucible with a little saltpetre. 

 The operation is very brief, the coloured external layer disappearing in one or two seconds. 

 This device involves no actual change of colour, but is simply a removal of the coloured 

 portions, which could have been effected just as well by the more lengthy process of grinding. 

 Attempts have been made, however, to decolourise diamonds in which the undesirable 

 pigment is distributed through the substance of the whole stone. Probably the first to 

 make experiments in this direction was the Emperor Rudolph II. ; according to the report 

 of his gem-expert, Boetius de Boot, the Emperor was acquainted with a method by which 

 every diamond could be decolourised, and rendered perfectly colourless. This important 

 secret, however, died with its possessor. At a later date the Parisian jeweller, Barbot, 

 claimed to be able, by the employment of chemical means and a high temperature, to 

 decolourise green, red, and yellow stones, while dark yellow, brown, and black stones only 

 slightly lost their colour. Barbot's method was also preserved as a secret, so that it is 

 impossible to put his assertion to the test ; it is probable, however, that he did not possess 

 the power to which he laid claim, in spite of the fact that he described himself on the titles 



