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Spinel Ruby

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RUBY.
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than those of the corundum, from which it differs in its composition only by having a lesser quantity of alumina.
It 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.
The 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.
They 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 transparent.
The 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
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