Sprinkled along the shores of the Great Lakes are clues into Michigan’s geological past in the form of stones. If you’ve ever harvested a handful of gravel from any of our state’s shorelines, then you know how varied these geological clues can be.

“A rock is a consolidated mass of one or more minerals,” explains geologist/paleontologist John Zawiskie, curator of Earth and Life Sciences at the Cranbrook Institute of Science in Bloomfield Hills. “The type of minerals you find in a given region is directly related to the nature of the geologic history, mode of formation of the underlying bedrock, and those exotic rocks brought by surface transport processes from other areas, such as flowing water, wind, and ice.”

The names of Petoskey, Isle Royale greenstone (our state’s gemstone), puddingstone, Lake Superior agate, and Leland bluestone — to name a few Michigan-based favorites — all reference regions, places, or time periods where the stones were found in abundance. As a result of glacial movement, the Michigan Basin (Lower Peninsula) has some of the most diverse mixture of minerals and rocks in the world, explains Zawiskie.

“Glacial erosion and transport over the last 2 million years have mixed up rocks from the various bedrock types,” he says. “The ice flowed such that the unconsolidated glacial sediments covering the Lower Peninsula now contain a mixture of igneous, sedimentary, and metamorphic surface rocks – arguably the most diverse assemblage of surface rocks in the world.”

All stones can be connected directly to a geological process, says Zawiskie. The texture of the stones in raw form gives hints as to how they were produced from the earth.

“Petoskey stones are fossil corals derived from certain layers of the sedimentary bedrock of the lower peninsula. Greenstone is a general term used to describe the weakly metamorphosed igneous (volcanic) rock basalt (crystallized from lava) or the hydrothermal mineral chlorastrolite, which forms from hot water solutions driven from magma into voids in basalt. These would be found in the igneous and metamorphic bedrock exposed in the western Upper Peninsula,” he explains. “Puddingstone is a metamorphosed sedimentary rock (quartzite with red jasper pebbles) and is found in the metamorphic bedrock cropping out in Ontario north of Georgian Bay.”

When it comes to collectible value, some of the stones are equally popular for their sparkle as they are for their symbolic meaning, explains geologist Ken Flood, owner and operator of Keweenaw Gem & Gift, Inc. in Michigan’s Keweenaw Peninsula. The area is famously known as a rock hound’s paradise.

“Greenstone is popular. It is rare. It is collectible as far as specimen material,” he says. “The rarity, tying into being the state gemstone, has an allure for people collecting it and using it in jewelry.”

Keweenaw Gem & Gift, Inc. prides itself on being “Michigan’s largest supplier of greenstone.” In addition to jewelry, Flood supplies stones in raw form to other area jewelers.

He adds, “We love to promote the stones from Michigan and especially anything that can be found up here.”

Not all of the Michigan “gems” are even stones, but they are popular because they give a nod to Michigan’s industrial past. One such example is copper brick, a mix of firebrick and copper that has combined and hardened from old smelting stations. Leland bluestone has a similar history. It is actually foundry glass or “slag,” a byproduct of iron from the Upper Peninsula smelted near Leland between 1875 and 1900. The worthless slag was dumped into Lake Michigan and shows up from time to time on local beaches.

Fordite, also known as Detroit agate, is yet another stone imitator. “Fordite is a cool product because it’s an automotive byproduct,” says custom jewelry maker and stone enthusiast Stephen Kolokithas of Jewelry Set in Stone in Chelsea. The name Fordite is a play on Ford Motor Company, but there are no direct ties to that company or to any other automotive manufacturer beyond the colors.

Kolokithas explains that, by the 1930s, the automobile painting process had evolved from hand-painting to spray-painting in order to increase efficiency, and the acrylic paint was then baked on to ensure a strong finish.  As paint colors changed for different cars, layers of baked paint would build up on the skids and tracks of the assembly line.

“They actually had people who would go into the assembly lines and clean the stuff off because it would build up so much that they wouldn’t be able to keep up production,” says Kolokithas. Today, the painting process in automobile factories has been refined to the point that there is hardly any overspray, which makes Fordite harder to source for commercial jewelry makers.

When cut and fastidiously polished, the reclaimed paint can glow like a pearl.  Psychedelic pastels are indicative of the 60s and 70s, while newer paints have metallic silvers, blacks, and blues.  “They’ve even found that Corvette red,” says Kolokithas. Fordite shines best when the purchaser can be involved in the process from start to finish. “There are a lot of different colors in Fordite, so you’re going to want to look for something that is appealing to you,” says Kolokithas. The right cut and polish will do the rest.