When sourcing Fuller's Earth, the most critical factor is the chemical composition, specifically the Montmorillonite content, which determines its absorption capacity. For cosmetic applications, ensure the powder is triple-sifted (mesh size 200-300) to guarantee a smooth texture. For industrial oil refining, focus on the decolorizing power and moisture content (typically <10%). Always request a Certificate of Analysis (COA) to verify the levels of heavy metals (Arsenic, Lead, Mercury), which must comply with international safety standards like USP or EP for personal care products.