than those of the corundum, from which it differs in its composition only by having a lesser quantity of alumina.
has single refraction like the diamond. It marks quartz, but is marked
by the corundum. It acquires electricity by friction ; does not melt
under the action of the blow-pipe, and in contact with acids does not
alter in the least.
Its specific gravity is 3-7 ; its hardness, 7-56. It is coloured by chromic acid.
The spinel is always found with the corundum, and it appears that both are produced by the same cause.
spinel is of various colours ; and in that section of the mineralogical
cabinet of the Roman university where the collection of the illustrious
Conte Lavinio Spada Medici is arranged above fifty different kinds of
spinelle, belonging to this eminent cultivator of mineralogical
science, are to be seen ; , there are some perfectly white and clear,
reddish-white, pale rose, and passing through all shades of wine-red
till they reach blackish-red.
are found in violet of every gradation of tint. In Greenland and at
Vesuvius they are found of amethyst colour. At Aker, in Sweden, as also
at Straskan, in Moravia, are found spinels of a blue colour, but not
ferriferous spinel, Cingalese, or black spinel, a variety which is very
small, bright, and black, is found in the lava of the Somma, in the
lands of Val