When discussing diamond characteristics, which of the following would be categorized as an inclusion?

The correct answer, which is categorized as an inclusion, refers to a cloud. In the context of diamonds, inclusions are internal features or imperfections within the stone that can affect its clarity and overall appearance.

A cloud is a specific type of inclusion that appears as a hazy area within the diamond and is typically composed of many smaller inclusions that are clustered together. This cloudiness can impact how light interacts with the diamond, thus influencing its appearance. Inclusions like clouds are typically taken into account when assessing a diamond's clarity during grading.

In comparison, surface scratches, polish lines, and crystals (which can be external or internal features) are not classified as inclusions. Surface scratches pertain to external blemishes on the diamond, whereas polish lines result from the polishing process and affect the surface rather than the internal structure. Crystals can refer to visible mineral inclusions that are trapped within the diamond, but they do not fit the broad definition of an inclusion the same way a cloud does. The categorization of clouds specifically aligns with the internal characteristics that define clarity and inclusion types in diamonds.