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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
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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
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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
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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
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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
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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
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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.
193:(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
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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
134:. 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
309:, 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.
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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.
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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.
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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
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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.
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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.
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Tumbling—Small gemstones are rotated in drums with progressively finer polishing grit for several days until they are smooth and reflective.
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284:. (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.)
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them, too. The advancing glacier acted like an enormous rock tumbler, abrading, fracturing, and rough-polishing the agates.
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can be used as a jewel when cut and polished. Only a fraction of the Lake
Superior agate is of the quality needed for
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color within or between an agate's bands. There can also be white, grey, black, and tan strips of color as well.
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low fluid pressure. The parallel nature of the bands also indicates the agate's position inside the lava flow.
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region. Finally, the Lake
Superior agate can be found in many regions of Minnesota as it was distributed by
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lodged in the lava flows until the next major geologic event that changed them and Minnesota.
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region. These flows are now exposed along the north and south shores of Lake Superior. The
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Lake Superior Agate from the Lake Superior region in Northern Minnesota.
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272:. Three lapidary techniques are used on Lake Superior agates:
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174:. Magma upwelled into iron-rich lava flows throughout the
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328:"An Introduction to the Geology of the North Shore"
237:The most common type of Lake Superior agate is the
352:Minnesota's State Gem: The Lake Superior Agate
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150:across Minnesota 10,000 to 15,000 years ago.
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138:Legislature as the official state gemstone.
51:introducing citations to additional sources
130:stained by iron and found on the shores of
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41:Relevant discussion may be found on the
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118:Lake Superior Agate - Duluth, Minnesota
178:, including what is now the Minnesota
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292:Distribution of Lake Superior agate
260:Lake Superior Agate set in 14k gold
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330:. Duluth Streams. Archived from
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16:Type of iron-stained agate
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176:Midcontinent Rift System
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252:Cutting and polishing
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62:"Lake Superior agate"
703:Geology of Wisconsin
698:Geology of Minnesota
693:Symbols of Minnesota
307:Northwestern Ontario
301:and the area around
47:improve this article
713:Geology of Michigan
664:Minerals portal
239:fortification agate
124:Lake Superior agate
718:Geology of Ontario
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559:Chrome chalcedony
533:Notable varieties
452:Cryptocrystalline
191:vesicular texture
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630:Herkimer diamond
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210:ensuing ice ages
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154:Geologic history
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334:on 8 July 2015
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132:Lake Superior
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126:is a type of
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58:Find sources:
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36:single source
32:This article
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635:Smoky quartz
412:Cristobalite
336:. Retrieved
332:the original
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103:October 2017
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564:Chrysoprase
519:Tiger's eye
399:Crystalline
303:Thunder Bay
217:Description
208:During the
682:Categories
645:Prasiolite
574:Moss agate
569:Heliotrope
541:Chalcedony
460:Chalcedony
437:Stishovite
432:Seifertite
313:References
199:chalcedony
180:Iron Range
144:Iron Range
73:newspapers
602:Geyserite
554:Carnelian
493:Fulgurite
485:Amorphous
442:Tridymite
282:cabochons
136:Minnesota
43:talk page
625:Ametrine
620:Amethyst
422:Moganite
391:minerals
270:lapidary
266:gemstone
184:tectonic
164:cabochon
597:Fiorite
417:Keatite
407:Coesite
338:28 June
197:called
87:scholar
688:Agates
612:Quartz
475:Jasper
427:Quartz
388:Silica
195:quartz
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549:Agate
524:Aqeeq
470:Flint
465:Chert
128:agate
94:JSTOR
80:books
589:Opal
579:Onyx
503:Opal
340:2015
297:the
122:The
66:news
305:in
49:by
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Text is available under the Creative Commons Attribution-ShareAlike License. Additional terms may apply.
