Besides the four Cs of grading diamonds and gemstones, there is a fifth factor that is important in grading sapphires - transparency. This describes the degree to which light can pass through an sapphire so that objects are visible through it. Transparency and clarity are related as inclusions can also block the passage of light.

The following terms are used to describe sapphire transparency:

Transparent
Transparent is given when objects seen through an sapphire look clear and distinct. Such sapphires usually have excellent brilliance, even if they might have some minor inclusions.

Semi-transparent
Semi-transparant is given to sapphires when objects look slightly hazy or blurry through the stone.

Translucent
Translucent is given when objects are vague and hard to see through the sapphire. Light can pass through, but it is somewhat diffused.

Semi-translucent
Semi-translucent or or semi-opaque is given when a small fraction of light can pass through the sapphire, mainly around the edges.

Opaque
For opaque sapphires, virtually no light passes through the stone.