Fuller's Earth Clay (montmorillonite) is a calcium-based clay that is fine, off-white, and highly absorbent. This natural mineral is extracted from the earth and was used as an important ingredient in a concoction used by wool textile workers to remove the oil from sheep's wool. It is absorbent enough to soak up auto oil and is also often used in kitty litter. Fuller's Earth is finer than other clays.