CORUNDUM (RUBY): CHARACTERS ^65 



Variety. Colour. 



" Oriental emerald " . . . green. 



" Oriental chrysolite ' 

 " Oriental topaz " 

 " Oriental hyacinth " 

 " Oriental amethyst " . 



yellowish-green, 

 yellow, 

 aurora-red. 

 violet. 



Ihe different colours of these varieties of corundum usually appear just as beautiful 

 when viewed by artificial light as by daylight, which is not always the case with the stones, 

 after which they are named. The colour of certain varieties of corundum can be in some 

 cases destroyed, and in others changed by the application of heat ; this point, however, will 

 be considered later on. 



The different colour-varieties of corundum occur as irregular grains and as well- 

 developed crystals embedded in the mother-rock, which, as a rule, is an old crystalline rock, 

 such as granite or gneiss. The gem-varieties are found with especial frequency as secondary 

 contact minerals, which have been developed in limestone by contact with a molten igneous 

 rock. By the weathering and denudation of such rocks, the embedded crystals are set free, 

 and are subsequently found with other water-worn debris in the beds of streams and rivers. 

 It is in these derived deposits that the most beautiful specimens of the varieties mentioned 

 •above are found in all countries in which the original mother-rocks occur. 



Having briefly considei-ed the characters common to all corundum, we pass now to the 

 consideration of the varieties suitable for use as gems, and begin with the most costly of all 

 ■stones, namely, the ruby. 



RUBY. 



Chaeacters. — Of all the colour-varieties of precious corundum the red, or ruby 

 ■('' oriental ruby "), is the most highly prized. It is probably identical with the anthrax of 

 Theophrastus, and is one of the stones referred to in mediaeval times as carbuncle. It has 

 -all the general characters of corundum, and is only distinguished from other varieties by its 

 red colour. A natural crystal of ruby is shown in Plate I., Fig. 5, and a faceted stone in 

 Fig. 6 of the same plate. 



The tone of colour differs in different specimens, being sometimes deep and intense 

 •(" masculine" ruby), sometimes pale and light ("feminine " ruby). The lighter shades vary 

 from pale rose-red to red dish- white, some specimens being so faintly tinged with red as to 

 appear almost colourless. The darker colours are either pure red, carmine-red, or blood- 

 red ; the red of the majority of rubies, however, has a more or less distinct tinge of blue or 

 violet, this being specially noticeable in transmitted light. The shade of colour which is 

 most admired is the deep, pure carmine-red, or carmine-red with a slight bluish tinge. 

 'This colour has been compared by the Burmese to that of the blood of a freshly-killed 

 pigeon, hence the references to such stones as being of " pigeon's-blood " red. The various 

 :shades of red of the ruby are remarkable in that they lose none of their beauty in artificial 

 light, a statement which cannot be made respecting any other precious stone of the same 

 ■colour. 



The colouring of rubies is not always perfectly uniform, colourless layers being 

 sometimes interposed between portions coloured red. In such cases, the stone will often 

 become uniformly coloured throughout after heating. Provided the stone is gradually 

 lieated it may be raised to the highest temperatures with no fear of fracture. The 

 interesting changes in colour exhibited by certain gems when gradually heated and then 



