Why is Brick considered a “natural” building material?

It's called that because brick is made from some of the most abundant, natural materials on Earth, clay and shale, then fired through a kiln to temperatures of up to 2,000 degrees. Brick's durability comes from the vitrification process in the kiln, which enables the clay particles to fuse together. Clay brick differs from concrete units that rely on a cement paste to bond materials together. Clay brick is significantly stronger than concrete brick.