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PRECIOUS STONES.
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in rolled pebbles mixed with gravel taken from the beds of
streams. It was used for jewelry in the Middle Ages, polished but
not faceted, as may be seen in the crowns of Lombardy and Hungary, and the crowns of the Gothic sovereigns and nobles which have been recently discovered near Toledo. Pure colorless sapphires are exceedingly rare, and sometimes are mistaken for diamonds, though the latter surpass
them in fire and lustre. Sir David Brewster believed the white variety, on account of its structure and refractive power, was superior to all other transparent minerals for the lenses of microscopes. This gem has been known to disclose a different color by natural light from that seen by artificial light ; that is, it may appear blue in the day, and purple in the evening, a phenomenon accounted for by supposing there exists an excess of red not visible by solar light. The green variety of the precious corundum, termed oriental emerald, is one of the rarest gems in existence, and when
of a lively green color, far excels the ordinary emerald in brilliancy and lustre. When it displays tints of sea-green or bluish green similar to those of the beryl, it receives the epithet of oriental aquamarine. Mr. Hamlin mentions some small, beautiful gems of this class obtained from the gold-fields of Montana. The purple or violet corundum, called oriental amethyst, also
very rare, seems to combine the hues of both the sapphire and the ruby. It is distinguished from quartz amethyst by its superior brilliancy, hardness, and beauty, though the latter is sometimes sold for the more valuable corundum variety. Some fine specimens are found at Dresden, and a few, with engravings, in the Vatican collection. The yellow corundum, denominated oriental topaz, is more
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