Virtually all diamonds possess natural imperfections, referred to as 'Inclusions' or 'flaws' which alter the overall purity of the stone. These are often difficult to detect with the human eye and for this reason a stone will be classified after being viewed under 10x magnification.
A stone is said to be "Flawless" if no inclusions are visible under a microscope. Truly flawless stones carry a high price owing to their rarity. The scale shown to the side is applied to stones when they are graded.
