Besides the four Cs of grading diamonds and gemstones, there is a fifth factor that is important in grading rubies - transparency. This describes the degree to which light can pass through an ruby 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 ruby transparency:

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

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

Translucent
Translucent is given when objects are vague and hard to see through the ruby. 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 ruby, mainly around the edges.

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