Umberstone is the main rock of the Erebus, which forms the base of the whole world. It is fancy, brown, and useful! Lots of ores can be found throughout the Umberstone, such as Jade Ore, Fossil Ore, Gneiss and all Vanilla Ores!
It can be used to craft Umbricks, Umbertiles, Umberstone Pillars, Umberstone Walls, ErebusCo. Smoothie-matic 2000s, Whetstones, Umbergolem Parts, and anything else that requires Smooth Stone, thanks to Forge's Ore Dictionary. For recipes, no doubt you're using Not Enough Items, so you know the drill.