Diamond clarity is the diamond quality that describes the existence and visual appearance of internal characteristics called inclusions as well as surface defects called blemishes.
Inclusions are natural identifying characteristics such as minerals or fractures that were naturally created while the diamond was forming. The majority of these natural birthmarks are invisible to the naked eye, yet they affect the way light is reflected and refracted within the stone. Trained professionals who use a magnifying loupe see these inclusions and classify the inclusions into three types: crystals, clouds, or feathers.
Inclusions are ranked on a scale of perfection, known as clarity. The number, size, brightness, nature, and position of inclusions define where the diamond falls on the clarity scale, ranging from F (flawless) to I (included). The scale is also based on the visibility of inclusions at the magnification of 10x.
Even with a loupe, the birthmarks in the VVS (Very, Very Slightly Included) to VS (Very Slightly Included) range can be very difficult to find. It is only when a diamond is graded 'I' that it is possible to see the birthmarks with the naked eye.
An inclusion in the middle or top of a diamond could impact the dispersion of light, sometimes making the diamond less brilliant, and generally affecting the value of a diamond more. There are very few flawless diamonds found in nature, making these diamonds much more valuable.