Quartz (SiO2) is clear and colorless, but a little bit of other elements in the mix can change the color drastically. Smoky quartz includes a bit of elemental silicon, making it black, and citrine’s yellow hue comes from iron contaminants.
Hemimorphite, Zn4Si2O7(OH)2·H2O, is named for the crystallographic phenomenon it exemplifies. A hemimorphic crystal is one whose opposite faces are symmetrically inequivalent.
Aragonite is a crystalline form of CaCO3, calcium carbonate.