Monday, November 22, 2010

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Minnesota's State Gem: The Lake Superior Agate

Geologic History

More than a billion years ago, the North American continent began to split apart into two separate continents. This catastrophic event, spurred by molten rock moving deep within the earth, poured out massive, iron-rich lava flows. These flows now are exposed along the north and south shores of Lake Superior.
The tectonic forces that attempted to pull the continent apart, and which left behind the lava flows, also created the Superior trough. The trough eventually became the basin of Lake Superior and the lava flows became the birthplace of Lake Superior agates.
Water vapor and carbon dioxide became trapped within the solidified flows in the form of millions of bubbles, called gas pockets or vesicles. Later, groundwater carrying ferric iron, quartz, and other dissolved minerals passed through the trapped gas vesicles. These quartz-rich groundwater solutions crystallized into concentric bands of fine-grained quartz called chalcedony.
Over the next billion years, some of these quartz-filled, banded vesicles -- agates -- were freed by running water and chemical disintegration of the lavas, since these vesicles were now harder than the lava rocks that contained them. The vast majority, however, remained lodged in the lava flows until the next major geologic event that changed them and Minnesota.
About 2 million years ago, the world's climate grew colder signaling the beginning of the Great Ice Age. A lobe of glacial ice, the Superior lobe, moved into Minnesota 10,000 to 15,000 years ago. It followed the agate-filled Superior trough. The glacier picked up surface agates and carried them south. Its crushing action and cycle of freezing and thawing at its base also freed many agates from within the lava flows and transported them, too. The advancing glacier acted like an enormous rock tumbler, abrading, fracturing, and rough-polishing the agates.

The Lake Superior agate differs from other agates found around the world in its rich red, orange, and yellow coloring. This color scheme is caused by the oxidation of iron. Iron leached from rocks provided the pigment that gives the gemstone its beautiful array of color. The concentration of iron and the amount of oxidation determine the color within or between an agate's bands.
The gemstone comes in various sizes. The gas pockets in which the agates formed were primarily small, about the size of a pea. A few Lake Superior agates weigh more than 20 pounds, about the size of a bowling ball. Such giant agates are extremely rare, but no doubt others are yet to be discovered.
The most common type of Lake Superior agate is the fortification agate with its eye-catching banding patterns. Each band, when traced around an exposed pattern or "face," connects with itself like the walls of a fort, hence the name fortification agate.
A common subtype of the fortification agate is the parallel-banded, onyx-fortification or water-level agate. Perfectly straight, parallel bands occur over all or part of these stones. The straight bands were produced by puddles of quartz-rich solutions that crystallized inside the gas pocket under very low fluid pressure. The parallel nature of the bands also indicates the agate's position inside the lava flow.
Probably the most popular Lake Superior agate is also one of the rarest. The highly treasured eye agate has perfectly round bands or "eyes" dotting the surface of the stone.
Occasionally, collectors find a gemstone with an almost perfectly smooth natural surface. These rare agates are believed to have spent a long time tumbling back and forth in the waves along some long-vanished, wave-battered rocky beach. They are called, appropriately enough, "waterwashed" agates.
Finally, the rarest Lake Superior agate is the one that recurs in a collector's dreams but is discovered in reality perhaps once in a lifetime. On average only one out of every 10,000 agates fits this description. They are the ones weighing 2 pounds or more and having perfect shape, color, and banding quality. They are the ones called "all-timers."
The word "gemstone" implies that a stone can be used as a jewel when cut and polished. The Lake Superior agate certainly qualifies, although only a fraction of the stones are of the quality needed for lapidary -- the art of cutting and polishing stones. During glacial movement, most of the agates were badly fractured by tremendous pressures within the ice and by repeated freezing and thawing.
Three lapidary techniques are used on Lake Superior agates. The most common technique is tumbling. Small gemstones are rotated in drums with polishing grit for several days until they are smooth and shiny.
Medium-size "lakers" (one-quarter pound to 1 pound) often are cut with diamond saws into thin slabs, which then are cut into various shapes. One side of the shaped slab is polished producing fine jewelry pieces and collectible gems called cabochons. Cabochons can be set in rings, bracelets, belt buckles, and tie clasps.
A technique called face polishing is less commonly used on the state gemstone. It involves polishing a curved surface on a portion of the stone and leaving the major portion in its natural state