Diamonds consist of only a single element, which is carbon. Intense heat and pressure over a long time turn the carbon molecules into a diamond. However, every diamond contains unique internal or external “birthmarks”, called inclusions (internal) and blemishes (external).
It is the inclusions and blemishes that make a diamond truly unique and unmistakable. Some diamonds come close to perfect, but most jewelers have never seen one.
The diamond's clarity, which measures the absence of inclusions and blemishes, is described by terms like FL (Flawless) for diamonds with no visible inclusions under 10x magnification, VVS for very slightly included or I for inclusions that are visible under the magnifying glass and may affect transparency and brilliance.
More on the clarity scale applied by the GIA.