Ahhh agates. Not so rare as you might think,
yet, quality specimens are coveted by collectors, jewelers and metaphysical
proponents. Collectors desire the multicolored pieces for the dramatic
display value, or historical or geographical prominence. Jewelers and
gemologists love the transparent/translucent beauty of the stone and the
fact that it takes on mirror like polish and can be worked with nearly any
method of lapidary. Metaphysicalists believe agates have profound spiritual
and physical powers.
Some of the older folks may have had their first contact with agate playing
the game of marbles. "Aggies" were once a prized addition to any marble
collection. At one time they were indeed hewn from actual agate. I have only
seen true "aggies" in museum collections. Any aggie you are likely to run
across now is going to be made of glass. Agate is a silica mineral of the
family Si02, or Silicon Di-oxide. (Glass is also a member of this group!).
Included in this group are agate's cousins, quartz, chalcedony and jasper,
among others. Agate and jasper differ mainly in the molecular arrangement,
which, in the case of jasper precludes the transmission of light, rendering
jasper opaque. Agate, on the other hand can vary from clear-as-glass
transparency to black-as-night opacity due to inclusion of trace minerals
that block the light. These trace minerals are what give agates their
coloration and personality.
I have a proclivity towards agates that are found in my native state of
Arizona. There is a wide variety of agate in nearly any color, transparency,
and texture scattered across the entire state, some of which are peculiar to
virtually only the state of Arizona (think "fire agate", which is really
chalcedony). Most rock collectors gravitate to the "famous" agate fields
near the towns of Quartzite, Brenda and Wickenburg or the 4th of July Butte
area. And, often they will tell you these areas have been "picked clean".
That may or may not be the case. Nice pieces are still found in these areas.
But, I have stumbled upon nice agates all over the state. Anywhere there is
volcanism, or a contact joint between hot magma and overlying rock (like the
line between granite bedrock and overlying metamorphic material), there is
the possibility of finding some kind of agate.
Basin, in south-central Arizona is famous for its Bloody-Basin Red Jasper.
However, there are agates to be found along Roundtree Canyon and along
Tangle Creek and even over towards the sheep bridge that spans the Verde
river. These agates tend to be pale clear-white in color with botryoidal
(smooth lumpy) texture. The layering is usually clearly visible along
fractured edges. You will probably encounter Indian ruins in this area. Try
not to disturb them, many times there are "watchers", usually retired or "",
ladies sitting somewhere with binoculars trying to catch folks "desecrating"
these sacred places. I don't know what all the fuss is about, as all these
ruins were thoroughly excavated by collectors in the 1920's and 1930's and
there is nothing left but stacked up rock walls and a few shards. They are
interesting to see, so leave them for the next visitor to appreciate.
Any time I see volcanic peaks composed of chunky black basalt, I immediately
think "agate". Especially in the fault-block part of the state (essentially,
the southern third). These volcanic formations must have been created during
a time with just the right combination of water and dissolved minerals to
create agates, jaspers and chalcedonies. The Brenda moss agate is scattered
along the east side of a large, long volcanic mountain (aka. Bear Hills)
north of I-10. The 4th of July Butte agates are a product of similar
volcanism. The butte itself being the "plug" left over after the rest of the
mountain has eroded away.
There are some nice agates near Perkinsville, north-east of Prescott, Az.
These are along several washes south of the Verde river canyon. Believe it
or not some of the best are right along side the main roadways in the area.
Sedimentary, Igneous or Metamorphic?
In the very end when an agate (chalcedony) is
finally created, it may have gone through all three methods in which rocks
are formed. Usually the process begins with an igneous environment whereby
hot fluids are being brought to the surface which carry with them the
necessary silica compounds of which chalcedony is made. Volcanism also
creates gas pockets and voids in which these fluids can penetrate and begin
the process of creating crystals. However it doesn't have to be particularly
hot- it is the gradual cooling which triggers the deposition of different
minerals in the formation of rocks.
Here is what my good friend says of agate:
Chalcedony (or agate) is the name given to the microcrystalline varieties
of quartz that form concretionary deposits (partially of organic origin in
the case of jasper). The word agate is used interchangeably with chalcedony
and is easier to pronounce. Chalcedony is formed in several environments,
generally near the surface of the earth where temperatures and pressures are
relatively low. It commonly forms in the zone of alteration of lode and
massive hydrothermal replacement deposits and as bodies of chert in chemical
sedimentary rocks. Chalcedony does not form crystals, but usually occurs as
crusts showing botryoidal and mammillary forms, also compact, banded. Due to
various inclusion, organic and inorganic, colors vary widely and give rise
to appropriate names; white to gray, brown, blue, black, and named varieties
such as: carnelian andsard, clear red to brownish red; sardonyx and onyx,
sard and carnelian in layers; heliotrope and bloodstone, green with spots of
red; agate, variegated, banded; moss agate, with mosslike or treelike
inclusions; crysoprase, apple-green; jasper, variegated and mottled red,
yellow, brown, tans; flint, whitish, dull gray, smoky brown to black