The Light Lumite Wall is a bright blue building-block with a violet slanted U-shape inside. 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 crafting-recipe has been unlocked upon discovering and reading the according Recipe Book (found in an Iron Treasure Chest in the Lava layer deep underground, or in a Diamond Treasure Chest in the Corruption layer very deep underground, or perhaps also as occasional pet-harvest from a Keepa).