MINERALOGY. 71 
6. Sapphire, Ruby, and Corundum. 
These beautiful gems are oxydes of aluminium, a silvery-white metal, and 
in chemical composition do not differ from the purest common clay, being 
rather its crystallization, modified by slight admixtures. The forms of the 
crystals are those of pl. 34, figs. 30, 36, 38. The most transparent blue 
crystallized variety is called sapphire, the yellow crystals are called oriental 
topaz, and the transparent red variety, oriental ruby. The less transparent 
kinds, of different colors, of green, blue, red, and greyish, are all grouped 
together under the general name of corundum. ‘The transparent varieties 
occur in single, frequently rounded, crystals and grains, in river beds, and 
in alluvial soils, especially in Ceylon, China, France, and Bohemia. 
Corundum is found in primary formations in Switzerland, North America, 
Sweden, the East Indies, and China. Red corundum is the ruby, which is 
often more highly valued than the diamond. Sapphires are very costly 
gems, and are generally cut like the diamond. Rubies of eight grains are 
worth from $200 to $250; the most esteemed sapphires of a dark-blue and 
pure color, are worth from $350 to $400 for a weight of twenty-four grains; . 
the paler, of sixteen grains’ weight, $25 to $30. The sapphire, ruby, and 
corundum, are the hardest stones after the diamond. 
7. Arsenious Acid. 
This has already been referred to under the head of arsenic. The most 
common forms of the crystals are the tetrahedron (fig. 39) and the 
octahedron (fig. 40). It frequently accompanies rocks containing arsenic. 
It is nearly allied to the following minerals : | 
8. White Antimony. 
This is an oxyde of antimony, corresponding to the last mineral, and 
occurs in crystals, whose form is represented in pl. 34, fig. 50. It 
accompanies the ores of antimony, and especially grey antimony. 
9. Red Copper Ore. 
This is an oxyde of copper occurring in beautiful crystallizations of the 
regular system, as in figs. 27, 33, 48, 44,61, 64. It is of a cochineal red color, 
passing into bluish grey, possesses an uneven conchoidal fracture, and an 
adamantine lustre on a very smooth surface. The finest crystals have been 
found in Cornwall, in Siberia, and at Chessy near Lyons, in primitive 
and transition rocks. 
Class 9. Silicates. 
Besides the simple oxydes just mentioned, there occur in nature a large 
number of their combinations, amongst which, those of different oxydes with 
silica (the silicates) are by far the most numerous. They are known 
before the blowpipe by their decomposition by the salt of phosphorus, a 
useful test, the silica being set free, and the base uniting with the free acid 
of the salt of phosphorus. The variety of silicates is very great, as will 
subsequently be shown. 
1. Datholite. 
Datholite is a combination of silicate of lime with borate of lime and water, 
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