A diamond is the hardest on the Mohs hardness scale which measures the hardness of a mineral by its resistance to scratching. But saying a diamond is the hardest is a little deceiving Hardness equals brittleness a diamond can be chipped or cracked very easily when faceted and mounted in a ring. Care for your diamond with regular cleanings and checking the prong tightness regularly by a jeweler. Take off your diamond when working with your hands in household cleaners or with tools .
when shopping for a diamond the best way to get exactly what you are paying for is to purchase a GIA certified diamond. The price of a GIA certified diamond will be a little higher than a diamond certified by other certification companies such as EGL, AGS ,HRD and IGI but GIA has been more accurate in certifying diamonds. Jewelers know they can look at a GIA certification and trust what the certification read. So when buying ask for a GIA certified diamond you will pay more but you will know exactly what you are getting.