Wabi Sabi is a Japanese aesthetic concept that values the beauty of natural roughness or imperfections in objects. There is beauty in a piece of rusted iron, for example, or an old gate with weathered wood. Acceptance of the impermanence and imperfections of things as a natural condition is an essential principle of Wabi Sabi. More than acceptance, imperfections are to be admired and appreciated for their beauty.
With diamonds, for example, the focus is on standardized measures of the four "C's": Carat (weight), Cut (e.g., brilliant cut), Clarity (freedom from flaws), and Color (white preferred). The closer these categories come to standardized ideals, the "better" and more valuable the stone. This way of evaluating discounts the uniqueness of imperfections and their unique beauty.
Antique diamonds, Old European Cuts and Old Miner Cuts for example, show the individuality of the human hand that cut and polished them, and show deviations from the perfection of machine cut stones of today. Since they score lower on the four C's, they are often dismissed as being inferior. Recently, however, many people are beginning to understand and appreciate these stones for their unique features, and they are becoming rarer. More on antique diamonds here.