Essentially - naturally occuring crystal.  Many semi-precius stones are versions of quartz with a specific set of impurities unique to that sub-stone.  Usually transparent to transluset, some quartz is solid and opaque.

Quartz comes in a range of colors from clear, with no flaws, to smoky pink, to forms of quartz that are defined by the flaws in the color - striping or banding that mars the clarity of the stone.