The 4Cs of the Diamond Quality Pyramid...

CARAT refers to the weight of a diamond. One carat is equivalent to 200 milligrams and can be divided into 100 points. A 0.75 carat diamond is the same as a 75-points or 3/4 carat diamond.
Since larger diamonds are found less frequently in nature, which places them at the rarest level of
the Diamond Quality Pyramid, a 1-carat diamond will cost more than twice a 1/2-carat diamond - assuming colour, clarity and cut remain constant.

CUT refers to the angles and proportions of a diamond. A well-cut diamond will internally reflect
light and disperse it through the top of the stone, resulting in a display of brilliance. Diamonds cut
too deep or shallow leak light through the side or bottom, resulting in less brilliance and value. Cut also refers to shape - round, square, pear, or heart for example. A round diamond is symmetrical, reflecting nearly all light that enters and making it the most brilliant of all diamond shapes.

CLARITY refers to inclusions in a diamond - natural characteristics such as minerals or fractures that occur while diamonds are forming. They look like tiny crystals, clouds or feathers and their position can affect the value and beauty of a diamond. Inclusions are ranked on a scale of perfection, known as clarity, ranging from F (Flawless) to Included (I). Some can be hidden by a mounting, but if in the middle or top of a diamond, they could impact the dispersion of light, making it diamond less brilliant.

COLOUR refers to the degree that diamonds are colourLESS and are graded on a scale from D (colourless) to Z. Warmer colours (K–Z) are particularly desirable when set in yellow gold while icy winter whites (D–J) look stunning set in white gold or platinum. Differences are very subtle and colours are therefore graded under controlled lighting and compared to a master set for accuracy. Like many things in life however, colour ultimately comes down to personal taste.