Diamonds and precious stones, a popular account of gems .. . e precious stoneswe are considering owe their remarkable colour, andconsequently a great part of their commercialvalue. But if the ruby, the sapphire, and kindred gems,are formed almost exclusively of alumina, we musthasten to add that this alumina is crystallized^ forin this fact is comprehended the cause of the enor-mous distance ivhich separates the alumina of thesoil around us from the alumina of which preciousstones are composed. CORUNDUM. Modern mineralogists have given the single namecorundum to all the minerals consisting of
Diamonds and precious stones, a popular account of gems .. . e precious stoneswe are considering owe their remarkable colour, andconsequently a great part of their commercialvalue. But if the ruby, the sapphire, and kindred gems,are formed almost exclusively of alumina, we musthasten to add that this alumina is crystallized^ forin this fact is comprehended the cause of the enor-mous distance ivhich separates the alumina of thesoil around us from the alumina of which preciousstones are composed. CORUNDUM. Modern mineralogists have given the single namecorundum to all the minerals consisting of crystal- CORUNDUM. 113 lized alumina almost pure, without regard to thecolour of these minerals. Corundum comprehends three varieties : the hya-line corundum, the laminated corundum, and thegranular corundum. The first variety comprehendsthe precious stones. The primitive form of the crystals of corundum isthe six-sided prism (Fig. 52), but the most generalform of the hyaline corundum is the dodecahedron(Fig. 5 3), with faces formed of isosceles
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Keywords: ., bookcentury1800, bookdecade1870, booksubjectgems, booksubjectprec
