Chests are found in both campaign and Horde. When you open a chest, it will randomly contain a gem, a journal (in campaign) or gold. If you find gold, it will be in increments of 500 or 1000. Note that you must complete the level in order for your gold to save. However, if you find gold in a chest that brings you over the 10,000 threshold mid-level, the achievement will unlock.