Water, a blue cube.
Water is one of the first materials you'll encounter, along with Copper . It can be mined from any freshwater source, but not from the sea (since you need freshwater, not salty sea water).
Water is a fuel type material, and so it's not used to build anything. Use water to power boilers on vehicles, and make sure you carry some water with you in case your vehicle runs out of fuel.
Things that can be fueled by Water