While Old Dog is right about what he says, a lot of the old mines were reopened and worked out. The Lost Shotgun Mine, The Dutch Oven Mine, Hank Brandt Mine, are some of the mines I know were found (in Cali). In AZ, there is the Lost Skull Mine (found by a friend of mine, and not what it was cracked up to be), I think the Opatas (Silver Mine reworked until played out by Mexican Miners), Pure Concepcion (Silver and Gold Mine found and mostly emptied in the late thirties, if you believe the story by Milton Rose).
Frank Fish found a strongbox (containing gold coin and ingots) and some large gold/silver balls around the Robber's Roost (near Inyokern, Ca).
Looking for the Lost Pearl Galleon in the Anza-Borrego Desert I think is a waste of time (unless for historical reasons). The ground is so acidic in Anza-Borrego, the pearls would have long ago dissolved.
There are several good gold and old weapons cache stories South of Interstate 8 (SoCal), but that close to the border is sketchy at best. BLM and Border Patrol usually don't allow driving anywhere near the border (in remote areas), and chances are good of you running into Illegals and/or Drug Mules in the area.
PegLeg's Black Nuggets are always a good look, and from the five or six known locations (approximate) of Black Nugget finds, you still have a good chance of hitting it big.
Most of the known stories have been hunted to death. Many of them are tall tales. Some of them are true, but have been found. And some are true, but the facts have been so twisted over the many years and many retellings, that any bit of original truth in them is long lost. ........and then, there are the few stories that have a basis in fact, and haven't been found (for whatever reason). They are few, but they are out there.
I have said it before, and I will repeat it here: "There are lots of unknown stories out there. The only way to find them out is to research libraries, and talk to some old timers, and Local Historical Societies. They will many times break a story to you that you have never heard before. I have a few like that. Sometimes they will give an old story a new twist, and give you a long missing clue."
Best,
Mike