The Red Mosaic Tile Wall is a rare terracotta-colored building-block with a dark red frame on two sides and dark red swung ornaments on four 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 by finding a rather rare Recipe Book in an Iron Treasure Chest on the Lava layer (ever since R22) underground, or in a Diamond Treasure Chest on the Corruption layer.