Like The Soldier Statuette, this level 14 item quest must either be done before Triss leaves Novigrad after Now or Never -- or, you can do this quest later, if you've successfully romanced her (ie, decided to be true to just her).

To get the quest, go to the merchant near the Gilford noticeboard near St. Gregory's Bridge. Buy the Jade Figurine from him, then go find Triss at The Cooked House or Rosemary and Thyme, head upstairs, and show her the statuette.

Triss will leave with you to case a spell, summoning a nobleman who was trapped in the statuette by Coral. That's all there's to it!