Since diamonds form under extreme heat and pressure, internal and external characteristics are common. These characteristics help gemologists separate natural diamonds from synthetics and simulants, and identify individual stones.There are two types of clarity characteristics: inclusions and blemishes. In order to grade the clarity of a diamond, it is necessary to observe the number and nature of external and internal characteristics of the stone, as well as their size and position. The difference is based on their locations: inclusions are enclosed within a diamond, while blemishes are external characteristics. IGI grading reports show plotted diagrams of clarity characteristics marked in red for internal and green for external features.