9 Rocks and Minerals Found In The Great Lakes

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The Great Lakes house a wealth of different minerals, stones, and fossils that are quite collectible. If you happen to live in the region it’s a good idea to know what’s out there, whether you’re a fan of agates or looking for the rare Michigan Greenstone. There truly is something there for every collector if you know where to look.

So, let’s dig into our list of rocks and minerals found in the Great Lakes.

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1. Lake Superior Agate

lake superior agates

There is perhaps no stone more closely associated with the Great Lakes than the Lake Superior Agate. These are red, orange, and yellow banded agates, quite different from those in the surrounding areas. These stones are closely associated with Lake Superior but can be found far south due to ancient glacial action.

Agates are a form of chalcedony, generally with banded staining or inclusions that resemble moss or plumes of smoke. Chalcedony itself is a cryptocrystalline mineral, meaning that it has no identifiable crystalline structure at our scale. It actually takes a powerful microscope and a thin section of chalcedony in order to study the crystal structure that makes them up.

The mixture of silica crystals that make up chalcedony are fascinating. The mineral is actually comprised of two crystalline silica forms intertwined, quartz and moganite. The latter is a monoclinic form of quartz, rather than the normal hexagonal structure associated with the mineral. Together these crystals form the waxy surface of agate, jasper, and chalcedony.

The main place to find them is on the shores of Lake Superior but they can also be found in the surrounding states, strewn across them by the painfully slow movement of glaciers millions of years ago.

2. Royale Greenstone

michigan greenstone

Royale Greenstone is a fascinating, and valuable, mineral. I’ve covered it in more detail before for those interested in finding samples. The mineral is actually called chlorastrolite, a variety of pumpellyite. It forms into small stones that have a turtleback pattern and often display chatoyancy. It’s the official gemstone of the state of Michigan.

Greenstone is very rare, especially in pieces larger than a few centimeters. Combined with its unique pattern, optical effect, and the ability to take a great polish it’s become a valuable gemstone in its own right. Stones larger than ½” across are considered very large, and the world record sample in the Smithsonian is only 3”x1 ½” across.

This stone is generally found along the shores of the Keweenaw Peninsula, the Upper Peninsula of Michigan, and along the banks of Isle Royale. Small pieces can be found in gravels, but they’re hard to identify in the field. These stones were formed in vesicles in the enormous basalt fields that lay under the Great Lakes, and this is the only place in the world they’re found.

While collecting from  Isle Royale National Park is now forbidden, you can still find some of this rare stone along the banks of the Keweenaw Peninsula. It’s not easy to find, but the rewards are substantial if you do get lucky.

3. Puddingstone

sample of a puddingstone

Puddingstone is a descriptor used for various conglomerates. The one found on the Great Lakes is a jasper and quartz conglomerate, creating a stone that has tiny pieces of jasper contained in a matrix with similar hardness. This jasper conglomerate is usually made up of red jasper and milky quartzite but other minerals are also found in it. They include hematite, black chert, and semi-transparent quartz.

Puddingstone is among my favorite stones to work with. The similar hardness of all of the minerals means that it actually cuts like a single material, with less fear about undercutting than you normally find in conglomerate material. The hardness of most of the minerals that make it up is 7.0 on the Moh’s scale, which means it also takes a great polish when finished.

Jasper conglomerate puddingstone can contain up to 90% pebbles locked in the matrix, but most pieces have less than that. Roughly 30% is standard, but each piece of stone varies even within itself. You never know what might be inside until you’ve cut it open!

They’re commonly found as glacial erratics across the bordering states but most of the good stuff is found on Drummond Island. 

Read More: Ultimate Guide to Puddingstones. What Are They and Where To Find Them

4. Petoskey Stones

round polished petoskey stone

Petoskey Stones are actually a fossil, formed from the ancient skeletons of an extinct family of coral. The coral, Hexagonoaria percarinata, was a horn coral. Often these stones are found with the edges of the coral ground off, which happened through glacial action as they were dragged out of the earth and tumbled along beneath the enormous weight of glaciers.

Many Petoskey stones are made into pieces suitable for jewelry. The unique pattern left behind is the fossilized skeleton of the coral, and it creates a roughly hexagonal shape of white that separates individual structures of grey-green stone. They take a great polish, and they’re highly sought after in the regions where they’re found.

Petoskey stones can be hard to recognize when raw. Anecdotal when a man found a 93-pound Petoskey stone and brought it home, someone else had to identify the stone for the officials who came to take it back. Turns out that state ordinances in Michigan prevent you from taking more than 25 pounds of material off the lake bottom. After a polish, however, they’re truly something special to look at.

Petoskey stones are found along the banks of the Great Lakes in general, but they’re particularly common on the beaches from Traverse City to Petoskey.

Read More: Petoskey Stone: Finding, Identifying, and Collecting Michigan’s Most Storied Fossil

5. Charlevoix Stones

petoskey stones

Similar to the Petoskey stones, Charlevoix stones are another form of fossilized coral. They look very similar, but the internal structure of the Charlevoix stones is smaller and the details often appear more intricate. They’re formed from the skeleton of a different kind of coral, specifically from the genus Favosite.

While quite similar, you’ll readily be able to tell the difference if you place a Chaelvoix stone next to a Petoskey stone. The latter has a much larger pattern overall, and Petoskey stones tend to be a bit less regular in structure as well. Charlevoix stones also lack the internal “wagon wheel” of the Petoskey stones.

When still alive, each hexagon in these stones bore a single polyp of coral. Remember that corals are animals despite the superficial similarity to plants. The skeletons they build from calcium carbonate are common finds, and give us a larger understanding of the ancient coral reefs of our planet. There are other types of fossilized coral to be found in this region as well, and the unique nature of the location (ie: the Great Lakes) means they tend to be rounded and polished before being found.

Charlevoix stones are found in the same places as Petoskey stones but are more widespread due to the coral lasting longer in the fossil record. 

6. Sodalite-Syenite

yooperlite

Sodalite- syenite is a fluorescent mineral that’s found on the shores of the Great Lakes. You’ll often see it referred to as Yooperlite, a trademarked name used by the first person to commercialize the stones. There’s a considerable amount of controversy over the various names the rock has been given, so we’ll stick with the actual classification of sodalite-syenite.

The sodalite contained in this stone is what gives it the glow underneath UV light, specifically it appears as orange spots on the background of the rock. Sodalite is a blue mineral that’s often cut on its own, whereas syenite is a classification of igneous rock. Most people seem to only have seen samples under UV light, in normal lighting these stones will appear white with blue spots.

Finding them is rather simple. You’ll want to go out to the location at night with a strong UV flashlight. Shine it in the water and along the shores and you’ll find some eventually, it sticks out well against the darker background. Finding it during the day is much harder, but it can be done if you learn to recognize the patterns.

Sodalite-syenite can be found on the shores of Lake Superior on the northern shore of Lake Michigan. Nighttime hunts are best, just make sure you have a good UV flashlight so you can easily recognize them.

7. Septarian Nodules

Septarian nodules have a strange story behind them. Originally formed as balls of clay on the floor of ancient oceans, over time the clay bonded with a mineral called siderite, before later cracking under the intense pressure of the Earth itself. The cracks were later filled in with calcite, usually yellow in color, and created a patterned effect on the stone.

Septarian nodules are strange in formation, but they come in an outstanding array of different patterns. In some cases the calcite doesn’t reach all the way to the surface, allowing smaller crystals to fill in the gaps in the clay-bonded siderite. In other cases the stone has been worn down by the Lakes over time, creating a smooth stone that appears to be separated by white or yellow bands.

The ones from the Great Lakes are often called “lightning stones” due to the way that the calcite grows through them. They’re a common find in the lower west side of Michigan, turning up along beaches in the area. They’re easily recognized by their tell-tale cracking.

The best spot to look for these stones is the western shores of Lake Michigan, but these fascinating concretions occasionally turn up across the whole region.

8. Jaspilite

Jaspilite

Jaspilite has many names, mostly depending on the whims of those who’ve pulled it from the earth. It consists of red jasper and black hematite in alternating bands. It’s very similar to the mineral called Tiger Iron and has roughly the same constituents. The difference is that Tiger Iron is folded in on itself and often has healed faults from ancient activity.

Jaspilite is beautiful, and it’s also used as a source of iron. The mineral is a rich ore of iron and important to industry in Michigan. It’s also cut into cabochons for jewelry by lapidaries, and sometimes even simply cut into polished slabs for display. It’s hard to turn down the aesthetic of black, grey, and red in intricate patterns.

Jaspilite is part of the massive family of banded iron formations. They’re usually defined by the way they’ve formed over time, with older samples often having been broken more than once. The formations in Michigan are fairly contained, but small pieces of it have been spread across the region by glaciers.

The best place to look is the aptly-named Jasper Hill, but it’s something to keep an eye out for if you’re rockhounding anywhere in this region.

9. Thomsonite

Thomsonite is a zeolite mineral, formed of small radial aggregates of crystals. The end effect is a stone comprised of eye-shaped spots, although it can also be found as thin crystals if they weren’t exposed to erosion over time. It’s a rare mineral, but it can be found in this region in pink and green coloration.

The nodules found around the Great Lakes are massive, meaning that they’re comprised of many different interlocking crystal systems. It’s rare to find crystals of thomsonite in this area. The massive formation and the natural motion of the lakes tend to leave behind tiny pebbles that display the tell-tale pattern associated with thomsonite from the Great Lakes.

Thomsonite’s value varies, but it’s an expensive stone in larger pieces. Pieces are graded on their coloration, with green being the most desirable and rare, as well as any internal folds or faults that have healed. Often thomsonite may have to be separated from the basalt bedrock in order to be worked, and many small pebbles can be found sitting in igneous rock along the lake shores.

The best place to look is on the shores of the lake between Lutsen and Grand Marais. More can also be found on the Keweenaw Peninsula. 

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