It’s the weight of the diamond; one carat’s divided into 100 “points,” so a diamond of 50 points weighs half (.50) a carat. Carat weight’s the most obvious measurement tool to determine the value of a diamond. Remember, two diamonds of equal carat weights can have very different prices, depending on their quality
Diamonds that are virtually free of flaws (also known as interior or exterior inclusions) are higher in quality because the absence of these flaws means there’s nothing to interfere with the passage of light through the diamond. To determine a diamond’s clarity, trained gemologists view them using a 10x magnification “loop.”
Color grading scales vary from totally colorless (D) to light yellow (Z); differences between one grade and another are very subtle and difficult to distinguish.
Lots of folks confuse “cut” with the shape of a diamond. Most diamonds are cut round with a full 58 facets; a good cut has more scintillation/more sparkle. But the shape of a diamond is more a matter of personal preference and doesn’t directly affect the value; the cut enables a diamond to make the best use of light.