Very good deals, seamingly open all the time.
Chabo's is in the basement of the Gift Market at 888 Brannan St, and though the shop is small, they have a good selection of rings, and can make just about anything. Rafi Chabo used to design for some Parisian designer (Dior?), but has the ability to recreate what ever you want. You can get a great deal on diamonds, engagement rings and wedding bands, and have anything else made for you.
In addition, they provide very good customer service.