‘Clarity' refers to the quality of the diamond, most particularly its flaw classification and it mostly affects the market value of the diamond. Only in the lowest grades of diamonds does clarity truly affect the brilliance of the stone, as brilliance is a matter of cutting skill and the chosen cutting technique. The clearer the diamond- the rarer and more expensive per carat weight it is. This is the sole reason any person purchasing a diamond must understand clarity, as with the case of color-grading, so that you may make your own evaluation on any given diamond and bargain different clarity grades for lower prices. Lower grade clarity diamonds that have been well-cut can as beautiful [or even more beautiful] than a clearer, poorly cut diamond.
This is also the reason we advice you to buy and practice using a loupe, as clarity qualification is based on what you see under 10x magnification, and with the loupe you may list the number of inclusions or blemishes in the diamond and familiarize yourself with them. Obviously this a task for a professional, but even an amateur in the field can see if a diamond is very low-grade and certainly flawed or cracked diamonds can be exposed with good lighting and a magnifying glass.