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

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

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

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

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