This is a sabotage quest given by Gil McMartin at -923, -48, 14 in The Commonlands. All of these quests take place within the City of Freeport. The quickest method to reach the city is by clicking on the manhole to The Thieves' Way at -1249, -84, 120 in the Commonlands. Inside the Thieves' Way use the grates to head into North, West, East or South Freeport.
You have 20 minutes to steal 5 crates of jewels. These are small light brown boxes with a slanted lid and are found around East, North, and South Freeport.
East Freeport: Around the shops and banker area on the docks