No two diamonds are alike. A diamond's most distinguishing characteristics are its inclusions, marks that are often invisible to the naked eye. However, under a jeweler's magnifying loupe or microscope they can look like crystals, tiny rivers, or clouds. A diamond's clarity is determined by the presence or absence of inclusions--fewer inclusions mean better clarity--and how visible they are. The greater a diamond's clarity, the greater its brilliance and value. A diamond categorized as internally flawless will have no inclusions, but this is extremely rare.
Though diamonds come in a wide range of colors, colorless diamonds have traditionally been considered the most valuable. Most diamonds are graded on a scale using the letters of the alphabet, from D (colorless), the best grade, through Z (a light yellow). It is difficult for the untrained eye to notice such variations in color unless stones are being compared side by side.
Diamond Carat Weight:
A diamond's weight is measured in carats, with one carat being equivalent to 100 points. You will often see a diamond referred to as a 3/4-carat stone or a 75-point diamond. Larger stones are often more highly valued, but size should not be the only consideration--high brilliance, which varies according to clarity, cut, and color grade, is highly desirable in a diamond.