Agate (Quartz:
chalcedony)
Agate,
is a conspicuously stripy variety of Chalcedony, It is an array of Quartz
gemstones poised of layers of quartz, at times of varying colors. When
it is concentrically banded (habitually in rather untamed patterns)
it is called by the sub-variety name "agate." When it is in plane layers/bands
it is addressed by the sub-variety name "onyx."Agate more often than
not transpires as rounded nodules or veins in rock such as volcanic
lava. These stratums of quartz are over and over again concentric in
nature. The concerto of agate changes significantly, but silica is always
principal, habitually with alumina and oxide of iron. Agates are formed
in most colors. There are quite a lot of varieties of agate. To name
the Common ones: blue lace agate, moss agate, tree agate and petrified
wood.
The
crypto-crystalline variants of quartz may be estranged into 2 types;
stringy and micro granular. Chalcedony is the universal term useful
to the stringy cryptocrystalline varieties. Agate is an illustration
of a stringy crypto-crystalline banded chalcedony range of quartz. Carnelian,
Chrysoprase and bloodstone are other chalcedony variants.
Types
of Agate
A
Mexican agate, showing only a single eye, has received the name of "cyclops
agate." Included matter of a green, golden, red, black or other color
or combinations embedded in the chalcedony and disposed in filaments
and other forms suggestive of vegetable growth, gives rise to dendritic
or moss agate (named varieties include Maury Mountain, Marston Ranch,
Sheep Creek and others). Dendritic agates have beautiful fern like patterns
on them formed due to the presence of manganese and iron oxides. Other
types of included matter deposited during agate-building include sagenitic
growths (radial mineral crystals) and chunks of entrapped detritus (such
as sand, ash, or mud). Occasionally agate fills a void left by decomposed
vegetative material such as a tree limb or root and is called limb cast
agate due to its appearance.
Turritella
agate is formed from fossil Turritella shells silicified in a chalcedony
base. Turritella are spiral marine gastropods having elongated, spiral
shells composed of many whorls. Similarly, coral, petrified wood and
other organic remains or porous rocks can also become agatized. Agatized
coral is often referred to as Petoskey agate or stone.
California's
"Mojave Blue" agate has gained a great deal of attention in the past
several years. This pastel blue or blue-gray agate cuts into attractive
cabochons for jewelry and, in the hands of an expert carver, makes outstanding
carvings.
Greek
agate is a name given to pale white to tan colored agate found in Sicily
back to 400 B.C. The Greeks used it for making jewelry and beads. Today
any agate of this color from Sicily, once an ancient Greek colony, is
called Greek agate. Yet the stone had been around centuries before that
and was known to both the Sumerians and the Egyptians, who used the
gem for decoration and religious ceremony.
Another
type of agate is Brazilian agate, which is found as sizable geodes of
layered nodules. These occur in brownish tones interlayered with white
and gray. Quartz forms within these nodules, creating a striking specimen
when cut opposite the layered growth axis. It is often dyed in various
colors for ornamental purposes.
Certain
stones, when examined in thin sections by transmitted light, show a
diffraction spectrum due to the extreme delicacy of the successive bands,
whence they are termed rainbow agates. Often agate coexists with layers
or masses of opal, jasper or crystalline quartz due to ambient variations
during the formation process.
Other
forms of agate include carnelian agate (usually exhibiting reddish hues),
Botswana agate, Ellensburg blue agate, blue lace agate, plume agates
(such as Carey, Graveyard Point, Sage, St. Johns, Teeter Ranch and others),
tube agate (with visible flow channels), fortification agate (which
exhibit little or no layered structure), fire agate (which seems to
glow internally like an opal) and Mexican crazy-lace agate (which exhibits
an often brightly colored, complex banded pattern) also called Rodeo
Agate and Rosetta Stone depending on who owned the mine at the time.
