any fine-grained, naturally occurring earthy substance that has a substantial ability to adsorb impurities or colouring bodies from fats, grease, or oils. Its name originated with the textile industry, in which textile workers (or fullers) cleaned raw wool by kneading it in a mixture of water and fine earth that adsorbed oil, dirt, and other contaminants from the fibres.

Fuller's earth consists chiefly of hydrated aluminum silicates that contain metal ions such as magnesium, sodium, and calcium within their structure. montmorillonite is the principal clay mineral in fuller's earth, but other minerals such as kaolinite, attapulgite, and palygorskite also occur and account for its variable chemical composition. Though similar in appearance to clay, fuller's earth differs by being more fine-grained and by having a higher water content. It also crumbles into mud when mixed with water, so it has little natural plasticity. The substance is found in a wide range of natural colours, from brown or green to yellow and white.

Fuller's earth is used to refine and decolourize petroleum products, cottonseed and soy oils, tallow, and other fats and oils. Its high adsorptive power also makes it commercially important in the preparation of animal litter trays and assorted degreasing agents and sweeping compounds. Fuller's earth usually occurs as a by-product of the decomposition of feldspar or from the slow transformation of volcanic glass into crystalline solids. Major deposits of fuller's earth have been found in England, in Japan, and in Florida, Georgia, Illinois, and Texas, U.S.