COLOURS OF PRECIOUS STONES 21 



called by a distinct name, and regarded popularly as a 

 distinct kind of stone. Radium rays can convert 

 colourless alumina or corundum into blue alumina 

 (sapphire) or red alumina (ruby), but they cannot 

 change alumina into beryllia (that is into emerald), nor 

 into fluoride (that is into topaz). 



One naturally asks, " To what is the colour of these 

 precious stones due ? "" The answer is difficult, because 

 very minute traces of chemical impurity, such as iron, 

 cobalt, manganese, or chromium may suffice to tint an 

 otherwise transparent, colourless crystal with the 

 brightest red, yellow, blue, violet, or green. Moreover, 

 it is certain from what we know of traces of metallic 

 impurity in artificial glass that it may exist in such a 

 state of chemical combination as to give no tint what- 

 ever to the glass, but after prolonged exposure to light 

 or other agencies, the minute impurity may combine 

 chemically with oxygen present in the glass and develop 

 colour. Thus, for instance, old window-glass often 

 assumes a violet or amethystine tint after long exposure. 

 This varying colour of the combinations of metals 

 according to whether they are oxidised or not, and the 

 degree of oxidation, or the special salt which they may 

 form, is in itself an unexpected thing to those who are 

 not chemists. The metal chromium, for instance, gives 

 rise to colourless, to yellow, red, green, and blue com- 

 binations. Manganese, a metal commonly associated 

 with iron, gives rise to brilliant green, to violet, and to 

 wine-red combinations, and if scattered as microscopic 

 particles of black oxide in glass would produce no colour 

 effect at all. From what we know of glass and the ease 

 with which it is coloured to every shade of the rainbow 

 by the admixture of traces of metallic impurities so 

 that " paste " or glass gems of all colours can be manu- 

 factured it is not surprising to find that natural 

 crystals, transparent and often devoid of colour (such as 

 corundum, diamond, quartz, and topaz), are yet also 



