The Stone Boulder Wall is a building-block looking like it was put together by rounded grey boulders on all 6 sides. It can be crafted in your Crafting Menu (default key "Q") or at Crafting Tables (that still exist in worlds created before R22 on September 16th 2015), but only after the recipe has been unlocked upon taking/crafting a Stacked Stone Wall for the first time, which is made from Stone or can also be found in Diamond Treasure Chests or looted / pet-harvested from Hot Feet, Silver Keepas, Golden Keepas or Things.