Knowledge. 



With which is incorporated Hardwicke's Science Gossip, and the Illustrated Scientific News. 



A Monthly Record of Science. 



Conducted by Wilfred Mark Webb, F.L.S., and E. S. Grew, M.A. 



JANUARY, 1915. 



THE HOME OF THE PIGEON-BLOOD RUBY. 



By LEOPOLD CLAREMOXT 

 {Author of the " Gem Cutter's Craft," " Ceylon, the Island of Jewels "). 



The mineral corundum assumes many aspects. 

 The vast majority of it is opaque and of a nonde- 

 script brownish grey colour, and this, on account 

 of the great hardness of the mineral, is used as an 

 abrasive material under the name of " emery." 

 For the same reason small plates of corundum 

 are used as the bearing parts of watches and other 

 mechanical contrivances. 



In comparatively rare cases, however, the crys- 

 tals of corundum are not only transparent, but 

 they are either colourless or they present in separate 

 specimens a long series of different colours of vary- 

 ing shades, which constitute gem-stones. 



Distinctive names are given to some of the 

 colours presented by the different varieties of 

 corundum, and thus the blue variety is called 

 " sapphire " and the red " ruby." 



For hundreds of years Burma has been famous 

 for producing rubies of the most desired colour. 



A perfect ruby is generally described by 

 connoisseurs of gems as being the colour of the 

 blood of the pigeon. It is, however, extremely 

 rareh' that a ruby comes to light to which this 

 description can with any amount of accuracy be 

 applied, so that those gems, the colour of which 

 more or less closely approaches this standard of 

 colour, are considered to be proportionately of 

 choice quality. 



It must also be pointed out that a shade which 

 may appear to be an approximation to the pigeon- 

 blood colour to the trained eye of one expert 

 may seem somewhat different to that of another ; 

 for, after all, the judging of rare gems is merely a 

 matter of opinion, dependent upon the indi\idual 

 appreciation of differences of shade. 



The composition of corundum is oxide of alu- 

 minium with traces of some metallic oxides, to which 

 the man}' colours presented by the varieties may 

 be attributed. Although its hardness is greatly 

 less than that of diamond, it is only surpassed in 

 this respect by that mineral. The specific gravity 

 varies from 3-94 to 4-08. 



Transparent corundum is doubly refractive of 

 light in all directions of the crystal except one, 

 which exhibits single refraction. It is also dichroic, 

 that is, it does not appear the same shade of colour 

 when \4ewed through different directions of a 

 coloured crystal. The crystals generally take the form 

 of six-sided prisms or double six-sided pjTamids ; 

 but these forms are in many cases greatly modified 

 according to the natural laws of crystallography, 

 and, moreover, the crystals are often found in a 

 fragmentary or water-worn condition. 



The ruby, or red variety of transparent corun- 

 dum, is derived from several Asiatic sources, the 

 gems from each of which present a shade of colour 

 which is peculiar to the area whence they are 

 derived, and by far the most important of the 

 sources is Burma. 



In the Natural History Branch of the British 

 Museum may be seen a fine example of a large 

 ruby in the natural or uncut condition. 



This gem, which weighs over one hundred and 

 sixty carats, was presented to the Museum by John 

 Ruskin in 1887 "in honour of the in\dncible 

 soldiership and loving equity of Sir Herbert 

 Edwardes' rule of the shores of the Indus." 



By permission of the Museum authorities a 

 photograph of the ruby, which is known as the 

 " Edwardes Rub}'," is reproduced in these columns 



