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parativelj low temperature at wliicli garnets fuse, since there does not appear 
to be any evidence that the garnets of the Kimberley Mines have ever 
attained this temperature. " One character," says Church (' Precious 
Stones,' p. 64), " common to all garnets save Uwarowite is their fusibility 
before the blowpipe ; they thus yield a vitreous mass which is of much 
lower density than the original garnet before fusion." This refers to experi- 
ments in air, of course, and things may be somewhat different under pressure 
deep down in the earth. In any event, the garnets of our mines have 
probably not been subjected to very high temperatures because none seem to 
have been found of the requisite low specific gravity. Kimberley garnets 
vary in specific gravity, it is true, but none tested in the De Beers ofiice have 
been anything like so low as 3, the value for vitreous garnet. We shall, 
doubtless, be on the safe side in assuming that the thermal expansion of 
garnet, zircon, etc., under temperature conditions that are reasonably 
probable, is always greater than that of diamond, though not necessarily 
greater by a constant multiple. 
Two alternatives are conceivable. First suppose a diamond to have 
crystallised about a foreign body, such as a garnet, and that at some subse- 
quent time there was an increase of temperature in the surrounding magma. 
It is clear that a greater relative expansion of the inclusion would put the 
diamond into such a state of strain that, if the magma were not solid, the 
diamond might give way and break. That there has been such an elevation 
of temperature is rendered not improbable by the large number of diamonds 
showing signs of corrosion (? Luzi's figures), and by the coatings of graphite 
on others ; possibly also by the phenomena of flaked diamonds. In the 
De Beers collection is one of these, a stone the greater part of which is in 
thin opaque grey laminae, like an oyster-shell.* The state of this diamond 
was for a long time erroneously attributed to shattering by dynamite, but it 
is more likely to have been caused by overheating in the pipe during some 
past time. Many Premier diamonds, also, show incipient local flaking 
("feathers") on their surfaces. Some of the diamonds belonging to the 
Emperor Francis I are said to have split into thin flakes when heated in a 
crucible. Elevation of temperature may also be indicated by the opaque 
white material, unaffected by hydrofluoric acid, occasionally found in local 
patches on diamond surfaces. Quite a large area of a certain lump of black 
bort from the Premier Mine was thus covered. A similar-looking stuff — 
perhaps the same — occurs, however, in some of the cavities in the cleavage 
faces of diamonds and round enclosed garnets, which makes it difficult to 
understand how it could be produced by heating ; for the heat required 
should presumably have destroyed the diamond surface outside before it 
could reach the cavity. 
* The crystalline part of this diamond is a non-conductor of electricity, but the 
laminated part conducts as well as bort. 
