NON-METALLIC MINERALS. 
ioa 
under the stipulations that all the property of the State should 
be confiscated to the East India Company, and that the Koh-i- 
noor should be given to the Queen of England. In order to 
bring out its lustre and remove some flaws, this gem was. 
re-cut after the Great Exhibition of 1851, under the direction 
of Messrs. Garrard, and its weight reduced from 186 y ^ carats, 
to 102J. 
In this Case will also be found glass models of the 
following celebrated diamonds, the weights of which are, 
where possible, expressed on the accompanying labels : — The 
Nizam ; the Mattan ; the Star of the South ; the Mogul ; 
the Begent, Pitt, or Orleans ; the Orloff ; the Tuscan ; the 
Florentine, or Austrian ; the Hope ; the Dresden ; the Nassuck ; 
the Pigof ; the Shah ; the Sancy ; the Eugenie ; the Polar Star 
the Cumberland ; the Stewart ; the first diamond found in S. 
Africa ; Mr. Dresden’s ; the engraved Persian ; and a diamond 
said to have been worn by Napoleon in the hilt of his sword. 
Some of these are, perhaps, of rather doubtful authenticity. 
The impure variety of diamond called bort or boort, which 
occurs usually in small spherical nodules, having a radiated 
internal structure, is employed, in the state of powder, as a 
grinding and polishing agent ; and much of the small splintery 
stuff from South Africa, is also called bort. The dark-coloured 
non-crystalline substance known as ‘ f carbon ” or carbonado is 
largely used for arming the crowns, or steel drill- heads, of the 
diamond-boring apparatus for rock -drilling, and for tools used 
in dressing millstones, &c. This peculiar form of carbon was 
discovered in the diamond gravels of Bahia, in Brazil in 1842,. 
and appears to furnish, in some of its characters, a transition 
from ordinary diamond to the next species — graphite. 
Case B. — Graphite, &c. 
The mineral known variously as Graphite ( graplio , I write), 
Plumbago {plumbum, lead), and “ Blacklead ” is composed of 
carbon and a small but variable quantity of iron oxide, and 
other impurities. It occurs in granite, schists, and crystalline 
limestone, as also in nodules in eruptive rocks. It is occasionally 
found crystallised in thin six-sided plates ; and a similar form 
is assumed by the artificial graphite or Kish formed during 
-certain metallurgical operations. Examples of this are exhibited. 
The plumbago from Borrowdale, in Cumberland, has long 
been celebrated for its fine quality, but the supply seems to be 
practically exhausted. The mineral was found in detached 
pieces, called, according to their size, sops or bellies. Plumbago 
is used largely in the manufacture of crucibles, and for the pre- 
paration oi blach lead pencils , as represented in this case. 
By Brockedon’s Patent here illustrated, the smaller pieces of 
plumbago are ground to an impalpable powder, which is sub- 
jected to enormous pressure, imparted by means of a screw press, 
