Clarity relates to markings that naturally occur within the diamond. The grading system starts at FL for flawless, to IF internally flawless, VVS very very slightly included, VS very slightly included, SI slightly included, and I included (also called pique). Diamonds are graded for clarity under 10X magnification. Most people cannot see flaws in a round brilliant cut diamond with the naked eye until the grading reaches SI2, I1 and below. The flaws may be black or clear coloured. The inclusions make each diamond unique, and are caused by small traces of non-crystallised carbon. Carbon is the element from which diamonds are formed.
Diamonds are graded under controlled conditions, with very white light and surrounds, with a comparative set of grading diamonds. As you will be viewing the diamond in a variety of different lighting conditions during daily life, not in a grading laboratory, you do not necessarily need a D coloured diamond to possess a beautiful white gem.