Check with the San Diego Chapter "Treasure Seekers". The area of Santa Rosa Mountain to San Jacinto mountain has been talked about in books as good areas for gems although most is taken up with 2 monuments now. The Wiley Well district just south and west of Blythe has lots of gem areas. Some well known is Houser and Potato patch geo beds, fire agate, chalcedony with a deep orange to chocolate color, amygdales, rhombs, jaspagate and agate. Some of this reaches into the Chuckawalla mountains (hills) and on the River Terrace area there is Fine quality agate, jasper and petrified wood. All of this is within the area reached by camping at the Willey Wells or Coon Hollow BLM campground. These are primitive campsites or camping is allowed on the open desert (dispersed camping).

The Oceanview and Pala Chief are great for gems also they are pay sights but found some great tourmaline, kunzite, and morganite.

Don’t take any big rocks to fill your bucket , move them out of the way and dig only dirt or you will not find anything
Everything is small and only in the dirt

I have to disagree about the big rocks. If you know what you are looking for the big rocks can be filled with stuff to find at home. I have found many a gem from there in the bucket of rocks I bring home and the ballls of red mud.