The Colored Gemstones Guide explains that topaz is formed through pneumatolytic processes in which hot gases play an essential role. It forms late in the cooling of a silica-rich igneous mass.
Pneumatolytic actions produce topaz, cassiterite, apatite and tourmaline, the Colored Gemstones Guide details. Topaz is typically found as a component of pegmatite dikes where crystals reach huge sizes. Additionally, it occurs in the cavities of gneisses, schists and highly acid rocks like rhyolite. Topaz is a common component in gem gravels because of its hardness and relative durability.
According to the University of California Berkeley, topaz crystallizes from fluorine-rich vapor in the final stages of solidification of igneous rocks. It often forms in cavities in granitic rocks and lavas, specifically in pegmatites and alluvial deposits. Secondary concentrations of topaz are found in stream beds and other alluvial deposits.
The Department of Geological Sciences at the University of Texas states that in gas cavities in volcanic rocks, topaz is typically associated with tin deposits. In pegmatites, it is usually associated with beryl, fluorite, tourmaline and apatite. Topaz is generally created as well-formed prisms with fair toughness. Most natural topaz is either colorless or has a very pale blue color. Irradiation, followed by heating, produces dark blue topaz.