224 Structure of some Precious Stones. [^Sl^SrS^ 
are on the wliole very free from crystals or cavities ; but I have 
seen a few fluid-cavities showing no facts of particular interest. 
Small hair-like crystals are common, arranged parallel to some of 
the planes of the crystal ; and sometimes the whole jargon is made 
up of alternating thin plates, like a complicated system of twin 
crystals, causing it to appear as if ruled all over with parallel lines ; 
which are more or less distinct and variously coloured, according as 
the plane of the plates is more or less inclined to the line of vision. 
The most interesting of the commoner objects seen in diamonds 
are the enclosed crystals. Brewster * was led to conclude that they 
were small cavities containing some elastic substance, which had 
exerted strong pressure on the surrounding diamond. I have, how- 
ever, been able to examine more and better specimens than he 
described, and find that they are really enclosed crystals, which have 
a strong depolarizing action on polarized light, giving well-marked 
colours, quite unlike anything due to mere cavities. Brewster has 
shown that the optical characters of diamonds are often similar to 
those of hardened gum, and this fact may often be made use of in 
recognizing that mineral ; but some of unusual beauty and purity 
are in a perfectly crystalline state, and have no more action on 
polarized light than any other crystals of the regular system. The 
minute enclosed crystals seem to have resisted the general contrac- 
tion of the substance of the diamond, and have impressed on it a 
depolarizing action, which gives rise to a black cross round each 
crystal. Fig. 9 ; whilst in other cases the tension has been relieved 
by the formation of cracks. Fig. 10. Similar phenomena may be 
observed in rubies, Fig. 11, and spinels, Fig. 12; and they will 
probably be found to be a common effect of the unequal contraction 
of minerals enclosed one within the other, whether large enough to 
be visible to the naked eye, as described by Mr. David Forbes,t or 
so small as to require the aid of the microscope. This explanation 
is strongly supported by what is seen in the case of crystals 
deposited in blow-pipe beads. By fusing zircon with borax, to 
which a good deal of microcosmic salt has been added, and keeping 
it hot over the lamp, cubic crystals of phosphate of zirconia are 
deposited in it. By placing the bead under the microscope whilst 
still hot, we can see that cracks form round the larger crystals as 
the temperature falls ; just as if they contracted less than the sur- 
rounding borax. Probably the further study of such facts will 
enable us to ascertain niore accurately the real temperature at 
which various rocks have been formed, and furnish another illus- 
tration of the value of the microscope in studying some of the 
greater problems of physical geology. 
* ' Trans. Geol. Soc.,' 2nd series, vol. iii., p. 455. 
t « Edinburgh New Phil. Journal,' July, 1857. 
