A few years ago we had a great PS get-together in the Union Square area. Went to Lang's, Cartier, de Beers, and a long-time local store whose name escapes me. We got special treatment in some places because there was a small gang of us, it would have been great even without that. I don't know of any special gem-related exhibits going on right now.
BTW you must check out the observation deck at the new de Young museum -- wow and wow!
I think Shreves is the store that VRBeauty cannot remember. SF Gift Show is going on at Moscone Center, but you need wholesale license to get in. Not jewelry related, but if you are interested in impressionist paintings, part of the collection from musee d"Orsay are being shown at deYoung.
AN
Yep, Shreve's is a nice choice. They also have a selection of vintage jewelry as well. And they're fairly close to Lang's. Granted, that area is pretty small...Also, hit Gump's, which is an old SF institution too. On their first floor they also have interesting jewelry, including some vintage.