You get this by doing the Joseph's Greenstone Ring quest.
His wife is located in Nashkel at coordinates (x 4224 y 2919). Find coordinates by pressing "x" key. The door to the house is locked, so you'll need to pick the lock or bash the door.
Talk to her and agree to look for the ring. Then, in the Nashkel Mines level 3, you'll find the ring. Its coordinates are (x 1596 y 260), but you'll see it right in front of where you appear when you get to the third level.
Return the ring for the achievement.