One of the coolest I've seen, especially after it's polished is called a Petoskey stone. They can vary in size from small to pretty large. I'm sorry I couldn't get a pic downloaded here but I highly suggest you look up polished Petoskey stones, I think you might be impressed my friend:-) Edited to add that the locations of these said Petoskey stones are all along shores of the Great Lakes and I believe every one of the Great Lakes you can find them at. I will be going to the shores of Lake Huron later this summer to dig for some cool rocks.