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GemBits - Chalcedony


#1

Silicon dioxide, better known as quartz, is one of the most abundant
minerals on Earth. Mineralogists divide quartz into two
classifications: crystalline and cryptocrystalline, based on the
size of its crystals. 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. Onyx, prase,
sard, sardonyx, and tiger’s eye are usually classified as jaspers.
The chalcedony family generally includes agate, aventurine,
bloodstone, carnelian, chrysocolla, and chrysoprase.

As described above, chalcedony is a group name. It also is the name
of a particular stone within that group. Blue Mist chalcedony is a
pale, almost white shade of blue that glows beautifully. It’s
durable and hard, making it a good choice for jewelry.

Agate probably has the greatest variety of color and variety of any
gemstone. It’s also one of the most abundant of all stones. Named
after the Achates River, which flowed through Sicily in ancient
times, agate takes a brilliant polish and has been widely used.
Archeologists have found agates used by humankind more than 20,000
years ago.

Most mineralogists require agates to show banding (stripes), however
many kinds of chalcedony without banding are commonly referred to as
agates. Dentritic agate, which appears to have ferns or tiny trees
enclosed within it, is one example. Moss agate and plume agate are
also unbanded chalcedony.

Ancient peoples ascribed both magical and medicinal powers to agate.
Placing an agate in the mouth relieved thirst, placing it against
the forehead relieved a fever, and wearing an agate helped its
possessor speak only the truth. Agate was also carved, especially
into bowls. European museums have extensive collections of these
bowls. Egyptians also carved agates into cameos more than 3000 years
ago.

Aventurine is another chalcedony that carvers used extensively for
bowls, vases, and ornaments. Although it is found in several colors,
aventurine is usually green. It’s been mistaken for jade. Most
aventurine sparkles, a phenomena known as aventurescence, due to
tiny bits of other materials embedded in it. Mica flakes create a
gold or silver glitter; goethite and hematite inclusions result in a
red or green sheen; and fushite is responsible for a glistening
green. As they did with most green stones, early peoples used
aventurine to soothe the eyes. It was also valued for its ability to
enhance the wearer’s mental acuity.

Like aventurine, bloodstone contains inclusions. Bloodstone, also
known as heliotrope, is usually green with red spots. Deposits of
iron oxide are responsible for the red, which does not glitter. One
legend maintains that the red formed when drops of Christ’s blood
landed on green jasper at the foot of the cross.

True to its name, bloodstone has a long history of healing bleeding
and diseases of the blood. People used it to stop nosebleeds and
bleeding from any kind of wound, by pressing it against the
afflicted area. Martyr’s Stone was another name given to bloodstone,
due to its extensive use by carvers to depict the crucifixion and
similar scenes.

Carnelian,also called cornelian, is a well-known red chalcedony.
Like bloodstone, its oranges and reds come from iron. However, the
iron is distributed throughout the carnelian, rather than
concentrated in spots.

Although used to stop bleeding, carnelian was more often worn to
prevent skin diseases and insanity. It was also reputed to bestow
courage and eloquence upon the wearer, and was especially
recommended for those speaking in public.

Chrysocolla is usually a wonderful robin’s egg blue, very similar in
appearance to the finest turquoise. Its hues can vary from
bluish-green to green. Copper is the source of the glorious colors
in chrysocolla. Pure chrysocolla is too soft to use in jewelry. Only
when it is formed in conjunction with quartz can it be utilized for
any purpose other than as a collector’s specimen.

Like most blue stones, chrysocolla is associated with peace and
calm.

Chrysoprase, like chrysocolla, incorporates the Greek word for
golden (chryso) in its name. Prase is from another Greek word
meaning leek, and the two names describe chrysoprase’s pale
yellowish-green color, a result of nickel impurities. Depending on
the mineral content, chrysoprase may also be bright green. Some
chrysoprase may be mistaken for jade.

Chrysoprase is the most valuable of the chalcedonies.

The chalcedonies are mined worldwide, including the United States.
The stones are generally cut into rounded shapes (cabochons) and
mounted in jewelry, or made into beads

****Sandra I. Smith, Writer ****


#2

Sandra: your article is very informative but your evaluation of blue
chalcedony is incorrect… Ellensburg blue- based on iron ion #4 is
the rarest of the chalcedony garnering $270 a carat for the gem
grade… it has a blue to gray blue color and has a unique red
cast… it will change color intensity with changes in body heat and
humidity… also it changes after you bring it down to sea level
unless it is a very stable piece… just another two cents for the
pot!..God bless America…ringman


#3

Ringman - while not doubting the rarity of the stone I’m realy
worried about the idea that it changes color, alters with air
preassure or has a red cast. Could you explain a bit more please.
I’ve never come across an agate that has these qualities and I’m an
experienced gemmologist. It’s a big gap in my knowledge!

Tony Konrath
Gold and Stone
www.goldandstone.com


#4

Hi: The agate is hydrothermic and has water trapped inside. Changes
in the water molecule with subtile heat and barometric pressure
acount for the varing degree of color intensity and hue. The rare
iron ion #4 accounts for the blue color and the red cast when you
past light thru it. If you would like I can send a sample for a
small cost, The stone is very much an orientation stone. That way
you get the best color. it seems that the hotter it gets the bluer
it will be…more to the gray when your body temp is down and there
is a heavy barometer… Unusual. yes. But this my experience with
the stone. Ringman… Been hunting it for 15 years