MK. J. T. IIOTBLACK ON PRECIOUS STONES. 
17 
We are fortunate in having on the table several specimens of 
the celebrated “ blue ground ” from the Kimberley mine, some 
belonging to the Museum, which were brought from the mine by 
Mr. Hackblock, one specimen of which shows a fair-sized diamond 
in situ, and which has the seal and a numbered mark of the 
De Beers Company, to show that it has been given by them. The 
other specimens of blue ground belong to myself, and were brought 
by me from the Company’s show at the Antwerp Exhibition, where 
they were showing the process of washing, and there is also on the 
table some of the gravel left after the washing, and from which 
most of the diamonds are picked. There are also found in the 
gravel very many garnets, in fact garnets seem always to be found 
in plenty with all sorts of other more precious stones. In this 
gravel, from which all the marketable stones have been taken, 
many small garnets may still be seen, and very small sparks of 
diamond may also be recognised. Perhaps I may here emphasise 
the fact, for I think there is no doubt that it is a fact, that even 
in this De Beers mine the diamonds are not in their true matrix. 
There is, I think, abundant proof of this in the many stones found 
already broken and in the worn appearance of nearly all. I take 
it that the blue ground is a kind of volcanic mud with which, or 
into which, the diamond has been thrown out of some airtight 
furnace of almost inconceivable heat, where the carbon lias been 
crystallised far down in the bowels of the earth. I could say much 
more about this wonderful stone, but the time at my disposal 
compels me to proceed, that I may give even a short account of 
some of the other “ gem ” stones. 
Corundum. 
The next I shall mention may very well be taken as an example 
of almost all the rest, as it is after the diamond more valuable and 
important than perhaps all the others. It is the stone generally 
called a ruby or a sapphire, according to its colour, but the proper 
name of which is corundum. It is one degree less hard than the 
diamond, which is reckoned ten, the corundum being nine in the 
recognised scale, the composition and specific gravity of the two 
stones also being very different. The S.G. of corundum is 3.9 to 
4.1 or 4.2. Its composition is about alumina 93.0, lime 1.0, 
c 
