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The minerals grouped as jasper belong to the quartz family. Quartz
is one of the most abundant minerals on Earth. It's formed primarily
from silica and oxygen, but includes trace amounts of dozens of
other minerals. Because of these mineralogical impurities, quartz is
found in myriad forms. Mineralogists have divided the quartz family
into two main groups: crystalline and cryptocrystalline.
Cryptocrystalline is also called microcrystalline quartz. Due to its
distinct crystal formation, crystalline quartz is usually
transparent. Rock crystal, amethyst, citrine, and rose quartz are
well-known examples of crystalline quartz.
The atoms in cryptocrystalline quartz pack together to form stones
that are either opaque or translucent. The atoms take the form of
fibers rather than crystals, and the stones often contain water or
air trapped between the layers of fibers.
The cryptocrystalline quartzes are informally divided into two
groups: jasper and chalcedony (kal-ced-nee). The jaspers are often
opaque, while the chalcedonies tend to be transparent. Most
mineralogists don't distinguish between the two, simply referring to
both as cryptocrystalline.
Because there are too many cryptocrystalline quartzes to describe in
one article, we'll use the commonly accepted groupings. The
chalcedony family generally includes agate, aventurine, bloodstone,
carnelian, chrysocolla and chrysoprase. Onyx, prase, sard, sardonyx
and tiger's eye are usually classified as jaspers.
Although not now regarded as precious stones, the chalcedonies and
jaspers were very highly valued in ancient times, not only for their
beauty, but also for their reputed magical and medicinal properties.
Picture, or scenic, jasper gets its name from the pictures of
scenes, animals or objects formed by the patterns in the stone. The
unique markings are caused by fossilized algae trapped within the
stone.
Ancient peoples used jasper to protect against pain, especially from
childbirth.
Onyx is a catchall term used to describe several stones. The pure
black stone often marketed as onyx is dyed chalcedony. The onyx used
in architecture isn't a form of quartz at all, but rather belongs to
the calcite family. Onyx agate is distinguished by parallel stripes.
It's often used for cameos, and it's the mineral described in this
article.
The stripes, or bands, on onyx are usually alternating black and
white. When properly cut, the concentric bands look like eyes,
leading most ancient peoples to use onyx to ward off the evil.
Although generally regarded as a protective stone, onyx gained a
reputation as a stone of discord. It was never worn by lovers, nor
was it worn at night, as it provoked nightmares. Prase, which means
leek in Greek, is a pale green form of jasper. Prase is often
included in the opal family, as one of the common opals. (Common
opals lack the flash and fire precious opals are noted for. There's
also a green stone recently discovered in Australia that has been
named Imperial prase by its discoverers. They chose that name because
it resembles chrysoprase. Their choice of name may or may not be
recognized by mineralogists.
Sard is a reddish-orangish brown stone that was often paired with
onyx by the ancients in the belief that the positive effects of red
would dispel the negative aspects of the black onyx.
Sardonyx is very similar to onyx in that it has parallel stripes,
and similar to sard in that the stripes are alternating
reddish-brown and white. People wore sardonyx next to their hearts
to heal depression. It was also thought to improve communication.
Tiger's eye starts life as crocidolite, an asbestos-like mineral.
Over time, quartz replaces the crocidolite fibers, creating a
silky-looking yellowish-brown gemstone. Tiger's eyes get their name
from their chatoyancy. Chatoyancy refers to the appearance of an eye
moving across the stone as it is rotated. The jasper family also
includes petrified dinosaur bone and petrified wood. In both these
instances, the original organic material was replaced by quartz.
(This process is also called fossilization.) The replacement
substance retains the same shape of the animal or plant. By its very
nature, petrified material is ancient.
Turritella is very similar to petrified dinosaur bones, in that it's
snail shells in which the original organic material has been
replaced by quartz. It's found primarily in the state of Wyoming.
All the jaspers are cut into cabochons (rounded shapes) rather than
faceted. Most are also made into beads or carved into ornamental
objects. Like the other members of the quartz family, jaspers are
found worldwide.
****Sandra I. Smith, Writer ****
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