From all groups of minerals the quartz-group is the one with the most varieties. The pure quartz is rock crystal, itīs just SiO2 (silica dioxide)
without any traces of colorgiving metal-oxides or other substances.
Other well known varieties are amethyst (violet), citrine (yellow), smoky quartz (brown, grey) rose quartz (pink), and less transparent stones like agate (any color) or jaspis (any color).
Also belonging to the quartz-group do stones like aventurine, chalcedony, chrysoprase, and opal, etc..
The hardness of quartz itsself is 7 but the quartz group reaches from 5,5 to 7.
Most of the gemtones are getting cheaper when clarity is not perfect, in quartzes this depends on the kind of inclusions, tourmaline or rutile needles can make a rock crystal much more expensive than a clear citrine.
A lot of interesting appearances can make a simple quartz really amazing: 3-phase inclusions with gas, liquid and solid substances, aktinolites, dendrites, hamatine, carbon, various metals and several others.
But also here fascination is depending on cut and polishing.
As optical effects there is asterism, chatoyance and various rainbow color appearances.
A physical effect is piezo electrism.
A nice effect is that quarzes do not attack your wallet like diamonds do.