How about Northern California's Gold Country? The 49ers did not get all of the gold. To this day, there are people who head out to tributaries of the American River to pan for gold, after a big storm has passed. (The Summer months are mostly dry, and a little too hot.) However I don't know any of the 'secret' places.
I have not heard anything about fossicking for gemstones locally.
Three potential hazards to be cognizant of:
•Paranoid cannabis farmers. That's why you should not hike off-trail in the foothills. (The high country is a different story.)
•Mountain lions and rattlesnakes. Take common-sense safety precautions.
•Poison oak. Learn what it looks like, and steer clear of it. It can cause very itchy rashes. But there's no poison oak locally above 4000 feet elevation.