Lake Superior Agates

By Alex Scott

lake superior agate collection

Lake Superior agates, Lane Baguss

The agate has been sought out and treasured by humans all around the world for thousands of years, since the Greek philosopher Theophrastus first found the rock in the Sicilian river Achates in the 4th century BCE and dubbed it agate in his text On Stones.

In addition to their historical use in laboratories and workshops as a very hard and resilient stone, their bright stripes and rich coloring make agates a coveted beach find. And none is quite so coveted as the Lake Superior agate, which Minnesota designated their state gemstone in 1969. This special rock is a must for any geologist’s or beachcomber’s collection, not only because of its unique coloring, but because of its equally unique history.

How agates form

The state of Minnesota choosing the Lake Superior agate as a symbol of their state is quite apt, as there is no better representation of the fascinating geological story of the Great Lakes area as the agate. Over a billion years ago, the Great Lakes area was part of a larger continent that would eventually become North America, called Laurentia. Due to the movement of the tectonic plates, Laurentia began to split apart, causing lava to erupt through the tears in the crust. As the lava cooled, it trapped gas pockets in the porous rock.

Over time, water rich in silica and various elements percolated through the bedrock and filled these cavities, creating striated bands of different minerals that stacked onto each other. These layers form the colorful stripes in agates.

how do agates form

But these stones were not yet ready to be collected by beachgoers (and not just because humans did not exist yet). For millions of years after Laurentia came back together and became the North American continent, the agates remained trapped within the layers of cooled lava.

This changed abruptly when the most recent ice age, which ended 11,000 years ago, caused the Laurentide Ice Sheet, a massive glacier that covered most of Canada and the United States, to recede towards the North Pole. As it did, it took the volcanic rock with it, scraping out the basins of the Great Lakes themselves and exposing many agates to the surface. With this massive upheaval, the agates were spread across the lakes and surrounding areas, waiting to finally be discovered. Over millennia, the baserock eroded away while the hard agates remained.

lake superior rock collecting

Left: Lake Superior agate in gravel pit (Jacob Boomsma). Top center: Agate on a Lake Superior beach (The Perfect Capture). Bottom center and right: Lake Superior agates (Ondrea Brussee).

Agates formed similarly all over the planet as plates shifted and volcanoes erupted, but in the Great Lakes area, it was predominantly iron that was trapped in the water that created the layers of the agates, giving them their signature rich red and orange colors.

Originally from Missouri, beachcomber Martha Jean Egelston now lives in Minnesota. She beachcombs along the north shore of Lake Superior, collecting Lake Superior agates and beach glass. The first time she saw Lake Superior, she was awestruck by its sheer size. “The lake is 34.9 miles long, 31,820 square miles, and maximum depth is 1,333 feet,” Martha says. “Approximately the size of South Carolina or Austria, many shipwrecks lie on the floor of the lake and big storms create 50-foot waves at times. It’s like being at the ocean except with no tides.”

great lakes rock hunting agates

Above left: Lake Superior (Martha Jean Egelston). Top right: Polished Lake Superior agates (Martha Jean Egelston). Above left: Lake Michigan geodes (Martha Jean Egelston). Above right: Lake Michigan gates (Tarah Hoffmann). 

Martha hits beaches from Duluth, Minnesota, up to the Canadian border, where agates are plentiful. “Agates are so common here that they can be found on gravel roads, anywhere there is excavating, homes being built, in riverbeds, and as filler rocks for flower beds,” Martha says. “You can find Lake Superior agates in most of Minnesota, Iowa, Upper Missouri, eastern Nebraska, Kansas, northwestern Wisconsin, and the Upper Peninsula of Michigan.” Her favorite agate finds are a two geodes, one with amethyst in the middle.

There are many different types of agates named for their location—like the Lake Superior agate—but they can also be classified by the type of striation that covers their surface. The most common type is the fortification agate, named so because of the concentric bands like the walls of a fort. Much rarer is the eye agate, which contains small concentric circles on the side of the stone that resemble a pupil and iris. These are formed when a crystal forms early in the layering process. Subsequent layers surround the “seed” crystal, forming a circle. An agate that has more than one “eye” on its surface is even more valuable. Lake Superior agates frequently contain numerous colorful eyes.

Tarah Hoffman collects agates along the beaches of Lake Michigan. She loves the beautiful layers in these rocks. “When agates are cut transversely, it displays the bandings and coveted eyes formed by nature during the cooling process.” Various agates are known to form in other ways such as the Lake Michigan cold water agates. “The Lake Michigan cloud agate is formed within limestone or dolomite strata and sometimes contains fossilized marine animals and corals,” says Tarah. “These agates are often less colorful than Lake Superior agates and resemble clouds over the lakeshore in their banded white, grey, and sometimes blue chalcedony.”

When looking at a Great Lakes agate it’s easy to see why it is so treasured by the beachcombers of the area, but its fascinating history is just as rich as its bright coloring. Next time you travel to the Great Lakes, be sure to keep an eye out for these precious rocks, only 1 billion years in the making.

Types of Lake Superior Agates

Lane Baguss is a photographer from Minnesota whose specialty is Quaternary geology, the study of the process and glacial till deposits that developed during the glacial-interglacial cycles of the Quaternary period. His Agate Variations Poster includes every kind of Lake Superior agate.

types of lake superior agates

lake superior agate types

agate types from great lakes

Learn more, purchase art prints, and follow Lane on Instagram and Facebook @lsagates.

This article appeared in the Beachcombing Magazine May/June 2020 issue.

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