Well-formed crystals typically form in a 'bed' that has unconstrained growth into a void; usually the crystals are attached at the other end to a matrix and only one termination pyramid is present.

However, doubly terminated crystals do occur where they develop freely without attachment, for instance within gypsum.

A quartz geode is such a situation where the void is approximately spherical in shape, lined with a bed of crystals pointing inward. Quartz is a defining constituent of granite and other felsic igneous rocks. It is very common in sedimentary rocks  such as sandstone and shale. It is a common constituent of schist, gneiss, quartzite and other metamorphic rocks.

Quartz has the lowest potential for weathering  in the Goldich dissolution series and consequently it is very common as a residual mineral in stream sediments and residual soils.

While the majority of quartz crystallises from molten magma, much quartz also chemically precipitates from hot hydrothermal veins  as gangue, sometimes with ore minerals like gold, silver and copper.

Large crystals of quartz are found in magmatic pegmatites. Well-formed crystals may reach several meters in length and weight hundreds of kilograms.

Naturally occurring quartz crystals of extremely high purity, necessary for the crucibles and other equipment used for growing silicon wafers in the semiconductor industry, are expensive and rare.

A major mining location for high purity quartz is the Spruce Pine Gem Mine in Spruce Pine, North Carolina, United States. Quartz may also be found in Caldoveiro Peak in Asturias, Spain.

The largest documented single crystal of quartz was found near Itapore, Goiaz, Brazil; it measured approximately 6.1×1.5×1.5 m and weighed 39,916 Kilograms.