There are many places to start. Any stream or river where you see gravel. The shores of the great lakes. Abandoned mines. Dry river beds. Exposed bedrock, fault lines, mines slag piles, your back yard. You can travel to places that are known gem producers.
To give you an example or two. I walk the beaches and streams around Michigan, we have mines and we are subject to many different rock strata around the lakes. We have the mountains where copper was formed and the ancient mines. The glaciers came through here, so I found a 300 lb. puddingstone in the yard. Imbedded in that is jasper and a lake superior agate. The upper great lakes there had significant volcanic activity, so there are basalt lava flows. The earths crust actually split and formed lake superior, lake ontario was also formed by a big crack in the earth's crust. Grab a rucksack and your rock hammer and go. In Ohio, there are sedimentary limestone bluffs along the highway where fossils are found. Quarries everywhere.
I literally love the Lake shores in Michigan. Surf, sand, ships, lighthouses, waterfalls and rocks like agates, jasper, jasperlite, amethyst, chalcedony, chert,