The Lake Superior agate is a type of
agate stained by iron and found on the shores of
Lake Superior. Its wide distribution and iron-rich bands of color reflect the gemstone's geologic history in Minnesota, Wisconsin, Nebraska, Iowa, Kansas, and Michigan. In 1969 the Lake Superior agate was designated by the
Minnesota Legislature as the official state gemstone.
The Lake Superior agate was selected because the agate reflects many aspects of Minnesota. It was formed during lava eruptions that occurred in Minnesota about a billion years ago.
The stone's predominant red color comes from iron, a major Minnesota industrial mineral found extensively throughout the
Iron Range region. Finally, the Lake Superior agate can be found in many regions of Minnesota as it was distributed by
glacial movement across Minnesota 10,000 to 15,000 years ago.
Geologic history
More than a billion years ago, the North American continent began to split apart along
plate boundaries
Plate tectonics (from the la, label=Late Latin, tectonicus, from the grc, τεκτονικός, lit=pertaining to building) is the generally accepted scientific theory that considers the Earth's lithosphere to comprise a number of large te ...
. Magma upwelled into iron-rich lava flows throughout the
Midcontinent Rift System, including what is now the Minnesota
Iron Range region. These flows are now 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, a depressed region that became the basin of Lake Superior.
The lava flows formed the conditions for the creation of Lake Superior agates. As the lava solidified, water vapor and carbon dioxide trapped within the solidified flows formed a
vesicular texture (literally millions of small bubbles). Later, groundwater transported ferric iron, silica, and other dissolved minerals passed through the trapped gas vesicles. These quartz-rich groundwater solutions deposited concentric bands of fine-grained
quartz called
chalcedony, or embedded agates.
Over the next billion years, erosion exposed a number of the quartz-filled, banded vesicles — agates. These 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.
During the
ensuing ice ages a lobe of glacial ice, the Superior lobe moved into Minnesota through the agate-filled Superior trough. The glacier picked up surface agates and transported 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.
Description
The Lake Superior agate is noted for its rich red, orange, and yellow coloring. This color scheme is caused by the oxidation of iron. Iron leached from rocks provides the pigment that gives the gemstone its beautiful array of colors. The concentration of iron and the amount of oxidation determine the color within or between an agate's bands. There can also be white, grey, black, and tan strips of color as well.
The gemstone comes in various sizes. The gas pockets in which the agates formed were primarily small, about 1 cm in diameter. A few Lake Superior agates have been found that are 22 cm in diameter with a mass exceeding 10 kilograms. Very large agates are extremely rare.
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 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.
Cutting and polishing
A
gemstone
A gemstone (also called a fine gem, jewel, precious stone, or semiprecious stone) is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks (such as lapis lazuli, opal, ...
can be used as a jewel when cut and polished. Only a fraction of the Lake Superior agate is of the quality needed for
lapidary. Three lapidary techniques are used on Lake Superior agates:
* Tumbling—Small gemstones are rotated in drums with progressively finer polishing grit for several days until they are smooth and reflective.
* 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. One side of the shaped slab is polished producing fine jewelry pieces and collectible gems called
cabochons. (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.)
* Face polishing—Polishing a curved surface on a portion of the stone and leaving the major portion in its natural state is called face polishing.
Distribution of Lake Superior agate
One of the most appealing reasons for naming the Lake Superior agate as the Minnesota state gemstone is its general availability. Glacial activity spread agates throughout northeastern and central Minnesota, northwestern Wisconsin, Northern Iowa, Nebraska, Kansas, and Michigan's Upper Peninsula in the
United States and the area around
Thunder Bay in
Northwestern Ontario
Northwestern Ontario is a secondary region of Northern Ontario in the Canadian province of Ontario which lies north and west of Lake Superior and west of Hudson Bay and James Bay. It includes most of subarctic Ontario. Its western boundary is the ...
, Canada. Lake Superior agates have been found in gravel deposits along the Mississippi River basin. Other types of agate similar to Lake Superior agate have been found in southwestern Wisconsin.
References
Minnesota's State Gem: The Lake Superior Agate
{{DEFAULTSORT:Lake Superior Agate
Agates
Symbols of Minnesota
Geology of Minnesota
Geology of Wisconsin
Geology of Iowa
Geology of Michigan
Geology of Ontario
Lake Superior