Most people are familiar with the petoskey stone, a fossilized ancient corral, hexigonaria percarinata, found along the shoreline of NW Michigan (first photo). There are many other fossils to be found on the lake, though. Check out the second photo. I found all of these on my Lake Trek.
There is also a wide variety of rock types and colors in Lake Michigan. Photos 3&4 are of rocks from SW Michigan shoreline. Photo 5 is from the final stretch from Milwaukee to Chicago. One even looks like a fossilized tooth (photo 6).
The final handful are not natural stones at all. This is cast-off slag (the impurities separated out during the refining process) from a steel mill that has been tumbled by the lake for decades.
Around the city of Leland, Michigan, these are called 'Leland Blues.' Local artisans there make earrings and other crafts using these intriguing 'stones.'
The colors of tumbled slag range from turquoise to milky blue and all shades in between.