The Four Cs of diamonds are the four characteristics that determine the quality and value of a diamond: cut, colour, clarity, and carat weight. The cut of a diamond refers to the angles and proportions of the diamond, which can affect its sparkle and brightness. The colour of a diamond is graded on a scale from D (colourless) to Z (light yellow or brown), with higher grades indicating a more valuable diamond. The clarity of a diamond refers to the presence or absence of blemishes and inclusions, with higher grades indicating a more valuable diamond. The carat weight of a diamond refers to its size and weight, with larger diamonds being more valuable.