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PRECIOUS STONES.
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structure
it shows a finely fibrous arrangement, which may be only seen under the
microscope, or may be apparent to the unaided eye. It occurs in rounded
masses (mam-millated or botryoidal), sometimes in stalactites, more
often in cavities in rocks.
The
history of its origin introduces us to so many features of interest in
the consideration of the origin of the large groups of minerals formed
by downward percolating water, that it has been described in detail
when speaking of the origin of precious stones in general.
Chemically
it is essentially silicon dioxide, but many specimens are found to
contain some water, varying in amount from practically nothing till it
approaches the quantity contained in Opal (q. v.). The general
appearance of the mineral differs somewhat with this varying
perÂcentage of water. Under the influence of solvents, such as alkaline
solutions, it shows a greater readiness of solution than Quartz. Bands
having different degrees of porosity are common.
It
is of very wide distribution in rocks of volcanic origin, which have
been subjected to some disintegration by water acting chemically. The
best specimens come from the Faroe Islands, Iceland and India.
Chalcedony
has been much used for vases, cups, beads, dishes, etc. When cut for a
ring stone or a brooch, it gives the finest surface if the cut is
across the fibre of the mineral. It is sometimes stained before
cutting, but not so frequently as the sub-variety Agate, hence the
staining will be considered under " Agate."
2. Carnelian.—This is Chalcedony coloured by the oxide of iron, Haematite ; it was the Sardius of old writers, and
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