The Stone Floor is a grey tiled building block that looks like it's made of 8 smaller stone blocks.
It can be crafted in your Crafting Menu (default key "q") after the according crafting recipe has been unlocked by
- crafting Wood Floor, or finding blocks of Wood Floor in Stone Treasure Chests
- crafting Stacked Stone Walls from Stone and Stone Rods, or finding blocks of Stacked Stone Walls in Obsidian Treasure Chests, or (rarely) looting / pet-harvesting them from Hot Feet, Silver Keepas, Golden Keepas or Things
To craft 8 blocks of Stone Floor at a time, you'll need (as of R44 in June 2017):