Carat is the universal measure of weight of a diamond and is equivalent to 0.20 grams. It is divided into 100 points. The name comes from the carob seed, which was used in the past by gem traders as counterweights in their balance scales due to the uniformity of their weight.
Clarity is the presence or absence of minute inclusions inside a diamond. (often thought of a unique birthmarks). It also refers to external marks and blemishes. As diamonds were formed over millions of years deep within the earth (under extreme pressure and temperature) they often have slight impurities that were caught up within the crystal during formation. Clarity is graded according to the visibility and type of inclusions ranging from the very rare and exceptional Flawless to Included, which has obvious inclusions that can affect the brilliance of a stone.
In colorless diamonds the most highly prized are those closest to colorless. The G.I.A (gemological institute of America) devised a universal system of color classification with the best D being colorless to increasing presence of color to the letter Z.
Cut is not only the physical shape of a diamond but also affects the three components that govern its beauty- brilliance, fire and life. Brilliance is the total light reflected in a diamond, life is the flashes of light or sparkle when a diamond is moved and fire is the dispersion of white light into the colors of the spectrum (the flashes of color we see in a diamond).
A diamonds proportions rule these aspects but its polish and symmetry are also important and affect how a diamond interacts with light. Certified diamonds are graded as Excellent, Very Good, Good, Fair and Poor.
Certificate is now widely regarded as the fifth C. A certificate is an evaluation of your diamond performed by a gemological laboratory. Each stone bears its own recognizable, individual characteristics, which is listed on the certificate. There are several recognized laboratories such as the G.I.A, HRD and IGI.