A Conglomeration of Aggregate

I don't know exactly what this stuff is, but I have been collecting it from the shores of Lake Erie for many years. It appears to be aggregate of some sort. Like lots of smooth beach pebbles held together with cement, broken into chunks and then tumbled for years in the Great Lakes.
If anyone has information on this stuff, I'd be very interested to know more. Many industries have dumped their slag and junk into our lake over the years and I imagine that these lovely rocky clusters I find are the byproduct of some manufacturing process.

I thought it might be fun to bezel set one of the smaller pieces of this stuff into a silver necklace. The back plate was made from the bowl of a sterling silver antique spoon. So upcycled and recycled all the way around. Something different. Find it here in my Etsy store.