PEESSURE CAVITIES IN TOPAZ, BERYL, AND DIAMOND. 41 



crowded group of small fluid cavities, none of which had entered it, — a proof that 

 the cavities had been formed in the topaz when soft, and when it imprisoned the 

 previously indurated crystal. 



The other class of phenomena to which I have referred is of a still more 

 remarkable nature, and has a more direct bearing- on geological theories. About 

 thirty years ago I communicated to the Geological Society the singular fact that 

 I had found in a diamond a small cavity, round which four luminous sectors were 

 seen in polarised light, — a phenomenon which clearly proved that the diamond, 

 when in a soft state, had been compressed by an elastic force proceeding from 

 the cavity. This inference countenances the opinion that the diamond was of 

 vegetable origin ; and as this gem was a sort of outlaw in the mineral world, the 

 idea that it had once been in a plastic state, like amber and other gums, and sus- 

 ceptible of compression, did not startle the mineralogists who believed in the 

 ordinary doctrine of crystallisation. The insulated fact, therefore, and the pro- 

 bable inference from it, excited no notice ; and it was not till the same pheno- 

 menon had been observed more frequently in the diamond, and in other mine- 

 rals supposed to be of aqueous formation, that its geological importance was 

 likely to be acknowledged. 



In the Koh-i-noor diamond, which the Prince-Consoet kindly permitted me to 

 examine in 1852, I found three black specks, scarcely visible to the eye, but 

 which the microscope showed to be irregular cavities, surrounded with sectors of 

 polarised light. In the two smaller diamonds which accompanied the Koh-i- 

 noor, there were also several cavities surrounded with luminous sectors, and the 

 same polarising structure which indicated the operation of compressing and 

 dilating forces.* In order to obtain more information on this subject, I examined 

 nearly fifty diamonds lent me by Messrs Hunt and Roskill, and in almost all of 

 them I found numbers of cavities, of the most singular forms, round which the 

 substance of the stone had been compressed and altered in a remarkable manner. 

 The shapes of the cavities sometimes resembled those of insects and lobsters, and 

 the streaks and patches of colour in polarised light were of the most variegated 

 kind. In examining a large number of diamonds, which adorn some of the 

 oriental objects in the East India Company's Museum, I found that all these 

 stones contained large cavities, and were coarse or flawed diamonds, which could 

 not be cut into brilliants, or used in rings or other ornaments. It seems, in- 

 deed, to be a general truth, that there are comparatively few diamonds without 

 cavities and flaws, and that this mineral is a fouler stone than any other used 

 in jeweller}^ Some diamonds, indeed, derive their black colour entirely from the 

 number of cavities which they contain, and which will not permit any light to 

 pass between them. 



* In 1820 I discovered similar cavities in amber, &c. See Edin. Phil. Journal, vol. ii. p. 334. 

 f See Edin. Trans. 1815, vol. viii. p. 157; or Journal de Physique, 1816, vol. Ixxxii. p. 367. 



