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160
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GEMS.
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Mexico, in a variety of trachytic porphyry, and in the Faroe Isles in amygdaloid.
Although
beautiful when just separated from the matrix, it loses much when
exposed to the damp and air ; when immersed in water, it comes out
quite transparent and dull, but it partly re-acquires its fire when
dried. Placed on the tongue, it leaves a disagreeable taste, by which
it is easy to distinguish it from other varieties.
When
looked at against the light it is quite transparent ; in the solar
rays it appears iridescent in a peculiar manner. Its specific gravity
is 3Ί2. By applying to its surface a thin layer of olive oil, its
brightness is increased, but only for a short time. It gives, on
analysis,
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The
common opal is found in great masses, in pebbles and in stalactites.
Its breakage is concave ; it is translucent and semitransparent. It is
found of various colours, milky, yellowish, whitish green, red, and
wine red. Sometimes it is dendritic. Its specific gravity is between
1-90 and 2-10.
It is composed of
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