Type of iron-colored agate found in the Lake Superior region
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Lake Superior agate is a type of iron-colored agate originating from the Lake Superior region in the United States and Canada. It commonly occurs as a collectible beach stone on the Lake Superior shoreline in Minnesota, Michigan, Wisconsin, and northern Ontario, and it can also be found in the interior regions of these and other Midwestern states. As a gemstone, Lake Superior agate is valued for its vibrant bands in shades of red and orange, but they may also white, grey, pale yellow, or brown. Believed to be the world's oldest agates,[4] Lake Superior agates formed within the basalticlava flows of the Midcontinent Rift System roughly 1.1 billion years ago. In 1969, the Lake Superior agate was designated by the Minnesota Legislature as the official state gemstone.[5]
A generalized geologic map of the northernmost segment of the Midcontinent Rift System. Lake Superior agates formed within several of the volcanic rock units (green, not labeled): the North Shore Volcanic Group (Minnesota), the Portage Lake Volcanic Group (Michigan's Keweenaw Peninsula and Isle Royale to the north) , the Osler Volcanics (Ontario, northeast of Minnesota), and the Michipicoten Island Formation (Ontario, east end of Lake Superior).
Lake Superior agates formed as nodules within iron-rich lava flows[6] that occurred roughly 1.1 billion years ago during the Mesoproterozoic era.[7] These lava flows resulted from the Midcontinent Rift, when the North American craton attempted to split apart along a rift2,000 kilometres (1,200 miles) in length that extended from present-day Lake Superior southwest as far as Kansas and southeast into Lower Michigan.[8] While volcanic activity occurred along the entire rift, Lake Superior agates only formed in the Superior trough, which formed the basin of Lake Superior when the last glaciers retreated from the area roughly 10,000 years ago.[6]
As the lava flows solidified into layers of basalt[b], bubbles of water vapor and carbon dioxide became trapped within the rock, forming vesicles.[6] Soon after,[10] silica- and iron-rich groundwater permeated the basalt,[6] forming a gel within the vesicles. Gradually, layers of chalcedony fibers and iron oxides develop from the gel, forming solid nodules of agate.[11]
Over the next billion years, some agate nodules were separated from their host rock by weathering. However, most remained embedded in the basalt until the ice ages of the Pleistocene epoch.[6]
During the Pleistocene glaciation, multiple glaciers descended from Canada and exposed the basalt, crushing it and freeing the much harder agates. The glaciers transported the agates across the Midwest, leaving behind rough-tumbled gravel deposits called glacial till.[10] Sources differ as to which specific glacial periods were responsible for eroding and redepositing the agates. According to one source, Lake Superior agates were named for a specific glacial deposit 1–1.8 million years in age called the Lake Superior Till.[12] Other sources implicate the last glacial period roughly 10,000–110,000 years ago,[6][13] and one specifically points to the Superior lobe that moved into Minnesota roughly 10,000–15,000 years ago.[6]
While many Lake Superior agates are found separated from their host rock due to glacial erosion or weathering, some can still be found embedded in the basalt. Lake Superior agates that have been subject to weathering or erosion are typically translucent and are less likely to contain inclusions, since the softer structures are easily broken apart. By contrast, Lake Superior agates still attached to their host rock are frequently opaque.[12]
Today, the remains of the basalt flows are exposed along the north and south shores of Lake Superior.[6] They comprise portions of the North Shore Volcanic Group in Minnesota,[8] the Portage Lake Volcanic Group in Michigan,[14] and the Osler Volcanics and Michipicoten Island Formation in Ontario.[15] Lake Superior agates have also been found in place in the Lake Shore Traps, a group of lava flows in the Keweenaw Peninsula that formed during a brief resumption of volcanic activity roughly 7 million years after the formation of the Portage Lake Volcanic Group.[16]
Cutting and polishing
A cabochon of Lake Superior agateLake 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:[6]
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 1/2kg 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
The localities in which Lake Superior agates originally formed include Michigan's Keweenaw Peninsula and Isle Royale, Ontario's Michipicoten Island, and the North Shore of Lake Superior extending from Duluth, Minnesota to Nipigon, Ontario.[17] Subsequent glacial activity spread the agates throughout northeastern and central Minnesota, northwestern Wisconsin, the Upper Peninsula of Michigan, central Iowa, eastern Nebraska and Kansas, northern Missouri, and the Lake Superior region of Ontario. Lake Superior agates have also been found in gravel deposits along the Mississippi River basin, some as far south as Arkansas and Louisiana.[18]
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.[19]
Notes
↑ The green representing the volcanic rocks in this formation is partially obscured by purple stars, which represent native copper deposits.
↑ Most Lake Superior agates formed within basaltic lava, but some formed in rhyolite.[9]
↑ Reed, R. C.; Daniels, Jennifer (1987). "Bedrock geology of northern Michigan". usgs.gov. Michigan Department of Natural Resources, Geological Survey Division. Retrieved 14 October 2025.
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