Peridot. Calm, uplifting, can be a little too "cloying" for people - it was described to me as someone telling you to smile - if you are truly upset about something it might just come off as annoying, but if you need a general pick me up it's excellent. It's green and resonates with the heart chakra. It's also a good general healing stone so if sadness is based in stuff you need to heal from it's good to have around, and if you have health problems it can be helpful.