Jewelry & Diamond Industry Dictionary of Terminology
An inclusion is a locational term, which is to say an inclusion is located inside a gem, whereas blemishes are found on the outside of a gem. There are many different types of inclusions, also known as internal clarity characteristics. As diamonds form in extreme heat and under great pressure, small crystals can become trapped inside. This is a common inclusion. Many of these crystals can only be seen if the diamond is studied at 10x magnification. If the diamond has an irregular atomic structure, it can also have a clarity characteristic called graining. Graining looks like faint lines or streaks. Diamonds can also have internal breaks, formed during or after formation. These breaks are called ‘feathers,’ because of their feathery appearance.
Sources for these definitions are GIA.edu, Google.com, Meriam-Webster.com, and Wikipedia.org.