Diamonds form under extreme heat and pressure deep within the earth. During formation the diamond crystal may develop some unique characteristics. Clarity is the absence of inclusions (internal characteristics) or blemishes (external characteristics). Clarity is graded from FL (flawless) to I3 (heavily included - very visible to the naked eye). Diamonds that are nearly perfect are known as flawless diamonds and they are exceptionally rare - even jewelers do not see them often, if at all.