(Note the value of large Lake superior agates, which weigh a few pounds or more, will lose most of their value if cut into slabs.) One side of the shaped slab is polished producing fine jewelry pieces and collectible gems called cabochons. Saw-cut and polish-Stones up to 1/2 kg are cut with diamond saws into thin slabs, which then are cut into various shapes.Tumbling-Small gemstones are rotated in drums with progressively finer polishing grit for several days until they are smooth and reflective.Three lapidary techniques are used on Lake Superior agates: Only a fraction of the Lake Superior agate are of the quality needed for lapidary. The highly treasured eye agate has perfectly round bands or "eyes" dotting the surface of the stone.Ī gemstone can be used as a jewel when cut and polished. Probably the most popular Lake Superior agate is also one of the rarest. The parallel nature of the bands also indicates the agate's position inside the lava flow. The straight bands were produced by puddles of quartz-rich solutions that crystallized inside the gas pocket under very low fluid pressure. Perfectly straight, parallel bands occur over all or part of these stones. Each band, when traced around an exposed pattern or "face," connects with itself like the walls of a fort, hence the name fortification agate.Ī common subtype of the fortification agate is the parallel-banded, onyx-fortification or water-level agate. The most common type of Lake Superior agate is the fortification agate with its eye-catching banding patterns. A few Lake Superior agates have been found that are 22 cm in diameter with a mass exceeding 10 kilograms. The gas pockets in which the agates formed were primarily small, about 1 cm in diameter. There can also be white, grey, black and tan strips of color as well. The concentration of iron and the amount of oxidation determine the color within or between an agate's bands. Iron leached from rocks provided the pigment that gives the gemstone its beautiful array of color. This color scheme is caused by the oxidation of iron. The Lake Superior agate is noted for its rich red, orange, and yellow coloring. Lake Superior Agate from the Lake Superior region in Northern Minnesota. The advancing glacier acted like an enormous rock tumbler, abrading, fracturing, and rough-polishing the agates. 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 glacier picked up surface agates and transported them south. The vast majority, however, remained lodged in the lava flows until the next major geologic event that changed them and Minnesota.ĭuring the ensuing ice ages a lobe of glacial ice, the Superior lobe, moved into Minnesota through the agate-filled Superior trough. Over the next billion years, erosion exposed a number of the 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. These quartz-rich groundwater solutions deposited concentric bands of fine-grained quartz called chalcedony, or embedded agates.
Later, groundwater transported ferric iron, silica, and other dissolved minerals passed through the trapped gas vesicles. As the lava solidified, water vapor and carbon dioxide trapped within the solidified flows formed a vesicular texture (literally millions of small bubbles). The lava flows formed the conditions for creation of Lake Superior agates.
The tectonic forces that attempted to pull the continent apart, and which left behind the lava flows, also created the Superior trough, a depressed region that became the basin of Lake Superior. These flows are now exposed along the north and south shores of Lake Superior. Magma upwelled into iron-rich lava flows throughout the Midcontinent Rift System, including what is now the Minnesota Iron Range region.
More than a billion years ago, the North American continent began to split apart along plate boundaries.
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