Transparency could be the degree to which light is transmitted through a gemstone.
The terms used to differentiate the amount of light transmission through a gemstone are:
Transparent. Light passes through relatively unobstructed. A graphic or object is seen through the gemstone without much distortion (except that as a result of facets or double refraction).
Semitransparent. Light passes through it is slightly diffused. A graphic or object is seen through the gemstone however with an outline that is indistinct slightly distorted.
Tanslucent. Light passes through but objects is not distinguished through the gemstone.
Semitranslucent. Light passes through only in the thin edges regarding the gemstone.
Opaque. No light is transmitted through even in the thin edges regarding the gemstone.