Agate

A small sample of agates from around the State of Arizona

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.

Bloody 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