Clarity indicates the presence (or lack of) inclusions inside the diamond, as well as external marks or blemishes. Most diamonds contain minute inner flaws or inclusions, often referred to as ‘birthmarks’, as they occurred during the long formation process of the diamond, over millions or billions of years under extreme heat and pressure deep within the earth’s core. Clarity is graded according to the visibility of inclusions or blemishes under tenfold magnification. The top grade, reserved for the rarest and most highly prized diamonds, is flawless (FL), showing no inclusions or blemishes. This ranges downwards through IF, internally flawless, meaning only blemishes and no inclusions; VVSI, meaning very, very slight inclusions, difficult to see under magnification even for an experienced grader; VS, or very slight; SI, slight; down to I, obvious inclusions which ultimately affect brilliance.