486 



THE GEOLOGIST. 



difference. Instead of a charcoal crucible, a crucible of alumina is 

 used ; and instead of a charcoal cupel for the boric acid, a platinum 

 one is preferred. The colour of the rubies thus obtained is identical 

 with that of the natural specimens. The rosy-violet tint so much 

 admired in the natural gem is present in the artificial one, where it is 

 due to sesquioxide of chromium. 



3. Sapphire.— The sapphire, or blue corundum, is produced under 

 the same circumstances as the ruby. It is also coloured with chromic 

 oxide, the only difference between them consisting in the proportion 

 of the colouring substance. Perhaps, also, there is a slight difference 

 in the degree of oxidation undergone by the chromium in the case of 

 sapphire production. But in such cases analysis can teach nothing, as 

 the quantities of colouring matter in either case is so extremely small. 

 In some cases, the authors of the memoirs alluded to above have 

 obtained, at the same time, crystals of ruby and crystals of sapphire, 

 the former of a red colour, the second of a magnificent blue tint 

 identical with that of the oriental sapphire. 



4. Green corundum. — When the quantity of chromic oxide in these 

 experiments is very considerable, the corundum obtained is of a brilliant 

 green colour, similar to that of Ouwarovite, a mineral to which we have 

 already alluded in the Geologist,"^' as containing 33 "5 per cent, of oxide 

 of chrome (according to Dr. Lewy and others) but which, from M. 

 Damour's analysis, appears to contain only 25 per cent. This green 

 corundum is always formed in that part of the crucible where the 

 fluoride of aluminium and the fluoride of chromium are placed ; the 

 latter becoming relatively more and more concentrated as the operation 

 proceeds (on account of its being the least volatile of the two com- 

 pounds), explains how it is that this green variety of corundum is 

 produced in this case in preference to the ruby or sapphire varieties. f 



5. Zircone. — Is produced by the same process as corundum (except 

 that fluoride of zirconium is employed in the place of fluoride of 

 aluminum). The zircone, or oxide of zirconium, shows itself in the 

 shape of minute crystals regularly grouped in arborizations not unlike 

 those obtained sometimes with sal-ammoniac. The chemical properties 

 of zircone thus obtained are the same as those of the natural speci- 

 mens ; but nothing is stated concerning the mineralogical or crystallo- 

 graphical characters. 



6. — The authors proceed to state that they have also obtained, by the 

 above method of experimentation, crystallized oxides of other metals, 

 e. g. of uranium, titanium, and tin. But neither their crystalline forms 

 nor their chemical composition have yet been investigated sufficiently to 

 be spoken of with certainty. 



7. CymopJiane or Chrysoheryl. — This mineral is an aluminate of 

 glucina, i.e. a compound of alumina and glucina. It is much prized by 

 jewellers, as it is capable of taking a fine polish, and produces a magni- 

 ficent gem when well cut. The Erench jewellers call it Chrysolite 

 orientale, whereby it is often mistaken for yellow corundum, which goes 

 under the same denomination. The colour varies from yellow to greenish 



* The Geologist, No. I., p 66. 



t All the above varieties of corundum are met with in nature. — T. L. P. 



