Where is the jeweler in Diablo 3?

The Jewler is located in ACT II, you first acquire the Jeweler during Act II, Quest 7, Blood and Sand.

You will first encounter the Jeweler during the Act II quest "Blood and Sand. In the sewers, you will find a Talking Barrel. Inside the barrel, you will find Covetous Shen. After freeing him, you will go on a short quest to recover an item the Crucible. After recovering the Crucible, Covetous Shen will set up in town similar to Haedrig.

You unlock the jeweler like you unlock the blacksmith, by doing the quest line.

The Jeweler is unlocked in the ACT II after the 7th quest of that ACT ("Blood and Sand").¹