For $70/person (as of 2/3/2010), you can screen for benitoite gems at this mine.  For more information you can visit http://www.calstategemmine.com/.

I don't know if you can find other gems there also.  However, on their website you can find imformation regarding accommodations, business days and hours, parking, supplies, rules, procedures, contact info, and sign-up.

From their website:

"From the westernmost stoplight in Coalinga, head west on Polk/Jayne Ave 1 mile.

Polk/Jayne make a large sweeping turn to the right, turing into Derrick. Follow Derrick for 3 miles.

Turn left on Los Gatos Rd, continue west exactly 15 miles. Turn in just before the white buffalo on the right-hand side and your there! "

They also have a map at this page: http://www.calstategemmine.com/Map.htm

The GPS Map coordinates are from mindat, but they are the actual mine not the screenings.   The reason why the screenings are moved is at this website: http://cash-and-treasures-wiki.travelchannel.com/thread/1438878/BLM+Closes+Clear+Creek+Management+Area%2F+Benitoite+Mine+still+open