Minerals!

Generally you can collect from Bureau of Land Management lands, just be certain to look up specific location rules. Don't confuse BLM land with National Parks though, collecting from NP's is illegal. There are also established public collection sites, but they aren't typically well known to the public as a whole. Local rockhounding/mineral clubs are a great resource too. They'll know about public sites, and usually list some information for non club members. If you join the club, other members might share some secret spots too!

I'm taking a trip to Nether's Farm in Ohio this September. It's a pretty unique location. It's a private property that sits atop the giant Flint Ridge flint vein. The farm owners allow public mining. You can bring out a generator and jackhammer and start your own pit, work an already existing one, or just rummage through other people's leavings! It costs $5 entry + 50¢ per lb. You just take your bucket up to the farm porch, weigh it, and slip your money envelope under the door.