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

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

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

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

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