their
deficiency in hardness, and increased specific gravity, at once
determine them. They arc also fusible in the blowpipe flame, while
quartz is infusible. Rock crystal has been employed very successfully
in the imitation of the ruby, sapphire, and other gems, by artificial
colouring,— first the stone is heated, and then plunged into prepared
solutions to give it the required colour.
The
cavities frequently found in quartz crystals have given rise to much
speculation and investigation, many of these cavities containing liquid
(these are Pliny's Enhydros or Enhygros). While at one time these
investigations were thought to lead to definite conclusions, as to the
origin of these lacuna, the results have not confirmed these
hopes. The contained liquid has been found to vary much ; in some it is
only water, sometimes water holding carbonic acid in solution,
sometimes liquid carbonic acid, while supersaturated solutions of
chloride of sodium have been found, minute crystals of rock salt being
visible within these cavities under the microscope, with fairly high
objectives. These cavities at times are full, others contain a bubble
which moves about on turning the crystal. But while so important from a
scientific point of view, they do not compare for ornamental purposes
with those specimens enclosing mineral crystals, such as fine fibres or
slender crystals of rutile, or other substances; these are known by
many fantastic names, as Loves Arrows, Cupid's Nets, Venus''s Hair, &c.; they are often cut for brooches.
In
America a good trade was at one time carried on in these gem minerals.
They are there cut into oval seals and charms, for use in jewellery.
These enclosed minerals are very plentiful in American crystals, while
in Japan and Madagascar large masses are found. They are not uncommon,
and are found in many parts of the world, including this Colony. In the
New England district crystals are often found enclosing cassiterite,
while in California they have been found enclosing stibnite, haematite,
dolomite, chlorite, and many other minerals, while hornblende is of
common occurrence.
"When
those specimens containing bubbles in the liquid of the cavities of
sufficient sizes to be readily seen, they are cut for gems, and sell
readily up to £5 each. They are often met with in jewels of the Cinque-cento period.
The
value of ordinary rock crystal in the rough varies much, according to
quality, but pieces of large size bring high prices. It is cut on a
copper wheel with emery, and polished with tripoli.
Natural
crystals are often so small that an ounce weight will contain between
7,000 and 8,000 stones, all of which are perfect, and doubly
terminated. In this colony these small crystals are often mistaken for
diamonds, but when seen under a lens they are found to be hexagonal,
while the diamond belongs to the cubical system.
When rock crystal varies in colour it has characteristic names, the yellow is known as citrine, or false topaz, the brown as Cairngorm, or smoky quartz, and the black as Morion.
The
remaining members of the quartz family will now be considered. The
principal of these are agate, amethyst, avanturine, cat's-eye,
chalcedony, chrysoprase, carnelian, jasper, heliotrope, milky quartz,
onyx, plasma, prase, rose quartz, sard, sardonyx, besides many
varieties of these. A collector may spend much time in making a
collection of pretty pebbles, but the one word quartz, with its distinctive names, will probably embrace the whole of them.
In
this colony large crystals are found in many localities, some obtained
at the tin mines of New England weighing nearly 1 cwt. It would be
tedious to enumerate the many localities whence rock crj'stal has been
obtained. Mineral enclosures have also been found at various localities.
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