imported>I2Overcome: added Precambrian, changed language, clarification on ice ages, added source

2025-03-07T10:59:28Z

added Precambrian, changed language, clarification on ice ages, added source

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{{Short description|Type of iron-colored agate found in the Lake Superior region}}
{{Infobox mineral
| name = Lake Superior agate
| category = [[Tectosilicate]] [[minerals]], [[quartz]] [[Mineral group|group]], [[chalcedony]] [[Mineral variety|variety]], [[agate]] variety
| boxwidth =
| boxbgcolor = #DC143C
| boxtextcolor = #ffffff
| image = File:Agat Lake Superior - Duluth, Minnesota, USA..jpg
| imagesize = 260px
| caption = Lake Superior agate from Duluth, Minnesota
| formula = SiO<sub>2</sub> ([[silicon dioxide]])
| molweight =
| color = Red, orange, pale yellow, brown, white, grey, rarely blue
| habit = [[Cryptocrystalline]] silica
| system = [[Trigonal]] ([[quartz]]) or [[monoclinic]] ([[moganite]])
| twinning =
| cleavage = None
| fracture = [[conchoidal fracture|Conchoidal]], with very sharp edges
| mohs = 6.5–7
| luster = Waxy, vitreous when polished
| refractive = 1.530–1.540
| opticalprop =
| birefringence = Up to +0.004 (B-G)
| pleochroism = Absent
| streak = White
| gravity = 2.58–2.64
| density = 2.6 g/cm³
| melt =
| fusibility =
| diagnostic =
| impurities = Iron oxides ([[goethite]], [[hematite]], [[limonite]])<ref name="lynch" />
| solubility =
| diaphaneity = Translucent to opaque
| other =
}}

'''Lake Superior agate''' is a type of [[agate]] found primarily near the shores of [[Lake Superior]]. It can also be found in interior regions of the [[United States|U.S.]] states of [[Minnesota]], [[Michigan]], [[Wisconsin]], [[Iowa]], [[Nebraska]], [[Kansas]], and [[Missouri]], and in the [[Canada|Canadian]] province of [[Ontario]]. As a [[gemstone]], it is valued by collectors for its vibrant, [[iron]]-colored bands in shades of red and orange, but they may also be white, grey, pale yellow, or brown. Believed to be the world's oldest agates,<ref>{{cite web |title=Lake Superior Agate |url=https://www.mindat.org/min-9253.html |website=mindat.org |publisher=Hudson Institute of Mineralogy |access-date=16 February 2025}}</ref> Lake Superior agates formed within the [[basaltic]] [[lava flows]] left behind from the [[Midcontinent Rift]] about 1.1 billion years ago. They are not named after the lake, but rather the Lake Superior [[Till]], a [[Pleistocene]] glacial deposit in which they can be found.<ref name="pabian">{{Cite book |last=Pabian |first=Roger |title=Agates: Treasures of the Earth |last2=Jackson |first2=Brian |last3=Tandy |first3=Peter |last4=Cromartie |first4=John |date=2016 |publisher=Firefly Books |isbn=978-1-77085-644-8}}</ref> In 1969, the Lake Superior agate was designated by the Minnesota Legislature as the official state gemstone.<ref>{{cite web |title=State Gemstone - Agate |url=https://www.sos.state.mn.us/about-minnesota/state-symbols/state-gemstone-agate/#:~:text=The%20Lake%20Superior%20agate%20was%20named%20the%20official%20state%20gemstone,iron%20ore%20in%20the%20soil. |website=sos.state.mn.us |publisher=Office of the Minnesota Secretary of State |access-date=7 March 2025}}</ref>

==Geologic history==
[[Image:Agatesuperiorminnesota.jpg|thumb|right|A [[cabochon]] of Lake Superior agate]]

1.109 billion years ago, during the Late [[Precambrian]], the North American continental [[plate tectonics|plate]] began to split apart along a 2000 km-long [[rift]] that extended from Minnesota southwest to Kansas and southeast to Lower Michigan. Instead of forming a new ocean, this process ended abruptly around 77 million years later for reasons that are not well understood.<ref name="Duluth Streams">{{cite web|url=http://www.lakesuperiorstreams.org/understanding/geology.html |title=An Introduction to the Geology of the North Shore |publisher=Duluth Streams |accessdate=28 June 2015 |author=Miller, Jim |url-status=dead |archiveurl=https://web.archive.org/web/20150708182221/http://www.lakesuperiorstreams.org/understanding/geology.html |archivedate= 8 July 2015 }}</ref> During this period, magma upwelled into iron-rich lava flows throughout the [[Midcontinent Rift System]], including what are now the [[Iron Range]] regions of Minnesota and Michigan's [[Upper Peninsula of Michigan|Upper Peninsula]]. These flows are now exposed along the north and south shores of Lake Superior. The Midcontinent Rift also created the Superior trough, a depressed region that became the basin of Lake Superior.<ref name="gem" />

As the lava solidified into layers of [[basalt]], bubbles of gaseous water and [[carbon dioxide]] became trapped within the rock, forming [[Vesicle (geology)|vesicles]]. Later, hot, silica- and iron-rich groundwater permeated the rock, forming a gel in the vesicles. Gradually, layers of chalcedony and [[Iron oxide|iron oxides]] were precipitated from the gel, forming solid [[Nodule (geology)|nodules]] of agate.<ref name="pabian" />

Over the next billion years, some agate nodules were freed from their host rock by running water and chemical disintegration of the lava rock, since agate is much harder than basalt. The vast majority, however, remained embedded in the basalt until the [[Ice age|ice ages]] of the [[Pleistocene]] epoch.<ref name="gem">{{Cite journal |last=Wolter |first=Scott F. |date=1988 |title=Minnesota Gem: The Lake Superior Agate |url=https://www.dnr.state.mn.us/education/geology/digging/agate.html |journal=The Minnesota Volunteer |volume=1988 |issue=Jan/Feb |pages=37-42 |access-date=2025-03-07}}</ref>

Between 1.8 million<ref name="pabian" /> and 10,000 years ago, during multiple periods of glaciation, [[Glacier|glaciers]] descended from Canada and exposed the basalt, crushing it and freeing the agates. The glaciers transported the agates across the Lake Superior region, leaving behind rough-tumbled gravel deposits called glacial [[till]].<ref name="lynch">{{Cite book |last=Lynch |first=Dan R. |title=Lake Superior Agates Field Guide |last2=Lynch |first2=Bob |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-282-6}}</ref> These deposits are the primary source of Lake Superior agates, although some can still be found embedded in their host rock.

==Cutting and polishing==
[[File:Lake Superior Agate Set in 14K.jpg|thumb|Lake Superior Agate set in 14k gold]]
A [[gemstone]] 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:<ref name="gem" />
* 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 [[cabochon]]s. (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==
Lake Superior agates originally formed along the Midcontinent Rift in Michigan's [[Keweenaw Peninsula]], Minnesota's North Shore, Ontario's [[Thunder Bay]] region, Lake Superior's [[Isle Royale]] and [[Michipicoten Island]], and in the area that has since become the bottom of Lake Superior.<ref name="lynch" /> Subsequent glacial activity spread agates throughout northeastern and central Minnesota, northwestern Wisconsin, the Upper Peninsula of Michigan, central Iowa, eastern Nebraska and Kansas, northern Missouri, and around the northern shore of Lake Superior in [[Northwestern Ontario]]. Lake Superior agates have also been found in gravel deposits along the Mississippi River basin, some as far south as [[Arkansas]] and [[Louisiana]].<ref name="pabian" />

In addition to the entirety of the Lake Superior shoreline, Lake Superior agates can be found on the shores of inland lakes, in gravel pits, along gravel roads, in riverbanks and beds, in farm fields, and in road cuts and embankments.<ref name="easy">{{cite book |last1=Magnuson |first1=Jim |title=Agate Hunting Made Easy |date=2012 |publisher=Adventure Publications |isbn=978-1-59193-326-7}}</ref>

==References==
{{Reflist}}
{{Silica minerals}}

{{DEFAULTSORT:Lake Superior Agate}}
[[Category:Agates]]
[[Category:Symbols of Minnesota]]
[[Category:Geology of Minnesota]]
[[Category:Geology of Wisconsin]]
[[Category:Geology of Iowa]]
[[Category:Geology of Michigan]]
[[Category:Geology of Ontario]]
[[Category:Lake Superior]]

Lake Superior agate - Revision history

imported>I2Overcome: added Precambrian, changed language, clarification on ice ages, added source