Open a Large Chest.
A Large Chest is a loot container that appears in certain areas. Opening a Large Chest causes a flurry of items to drop out.
Large Chests can only be found in the unique maps Vaults of Atziri and Acton’s Nightmare.
There is also a chest in The Twilight Strand (Act 1), which contains the first support gem for a new character. Despite the chest named Large Chest, it has different look as well as did not register for the achievement.