Rock crystal, one of many types of quartz crystals (SiO2), is a type of gem. When rough rock crystals are combined with pearlash at a glass furnace, it can be used to make crystal glass items. Cut rock crystals can't be used for this purpose.
Crystal glass is slightly complicated to produce, but can also be cut into gems with five times the material value, made into crafts and furniture that will keep your dwarves happy, and even utilized as a magma safe building material.
[SOLID_DENSITY:2650] Common to quartz gems