Rivers sort; roads expose
River gravels and gravel bars deliver the widest variety — the river has done the collecting from upstream. Road cuts and construction sites expose fresh rock faces (ask permission; rockhounding on private land without it is trespassing). Beaches sort by density — magnetite sands, agate pockets, and wave-tumbled quartz.
The legal layer
Public lands vary: national forests often allow reasonable personal collecting; national parks prohibit it absolutely; state rules differ by state. The BLM and Forest Service publish their rules; the one-sentence version is "check before you collect, and never sell what you find on public land without a permit."
Safety notes that are not decoration
Rock cuts collapse; mines are structurally treacherous and legally hazardous; and fibrous minerals (some serpentines) should never be powdered or breathed. Wear eye protection when hammering, wet-wipe suspicious fibers, and wash hands after the field — the hobby is safe when its edges are respected.