A natural is a bit of the naif or outer skin of a diamond that remains after cutting and polishing. A natural on the girdle shows that the cutter maximized the rough when mapping out the facets. Rarely their naturals can be found on a facet. Because they are the outer skin of the diamond, features found on a natural can also helpful for gemologists in diamond identification. A triangular feature called a trigon found on the outer skin of a diamond is a proof that the stone is natural.
Trigons on the Face of a Natural Diamond.
Photo Courtesy of Gump Stump, Wikipedia.