Transparency.
Reading edition
Precious stones are, on the basis of their relative transparency, divided into four classes, as follows: Transparent, or admitting light freely and clearly; defining objects when used as a lens. Semi-transparent, admitting light, but only partially defining objects. Translucent, admitting light faintly. Opaque, not admitting light.
The more valuable precious stones, excepting opals and turquoises, are generally transparent.