Rock crystal is the name given to all clear colorless quartz.

It commonly occurs in quartz veins where it crytallizes inside rock cavities known as vugs. It also is common in vugs or pockets in pegmatite dikes. Rock crystal often occurs as secondary quartz crystals on cryptocrystalline quartz in cavities and vugs, and in geodes. Rock crystal often has minute cavity inclusions containing carbon dioxide or water. If there is a sufficient number of these minute fluid inclusions then the quartz becomes opaque and is called milky quartz. Most rock crystal forms on a matrix of milky quartz and crystals of rock crystal often grade into milky quartz towards their base. Rock crystal often has cavities containing both liguid and gas phases known as a two-phase inclusions. In rare cases, two-phase inclusions in quartz can be large enough to be visible to the unaided eye and may show actual movement of the gas phase within the liquid phase.

The best rock crystal sources are in the famous Hot Springs area of Arkansas, USA; Cumberland, England; St. Gotthard, Switzerland; Brazil and Madagascar.Large individual crystals of quartz have been found in Brazil, the largest weighs over 44 tons.