All buildings are made of earth, one way or another. No one has ever built anything whose materials and products didn't come out of the ground. It just takes a bit more processing to get, say, structural steel or an LED light bulb than to make an adobe brick. Low carbon or low-impact building means choosing products and materials that provide the desired roles and functions with a minimum of energy, water or chemical fuss.
Design comes first. That is, picking products and materials comes after - not before - you've clarified the purpose, massing, orientation, fenestration, and other fundamental qualities of a building project. And good design is not just about beauty and looks; it is built on a foundation of good science, or else it will soon be feeding mold and lawyers. Here are some great resources on moisture, insulation, durability and all things building science presented in a way that anyone can understand: