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.