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A clear, glassy, mineral with conchoidal fracture (no cleavage). When quartz grows in a free space it forms a six sided crystal with pointed ends. But because quartz is the last mineral to crystallize from a magma it grows to fill the spaces remaining between the other crystals and in rocks typically shows no crystal shape.
Quartz is composed of all silica tetrahedra where all oxygen atoms bond covalently with all silica atoms. This makes a very strong crystal, but with no planes of weakness. Thus quartz does not cleave.
In felsic igneous rocks quartz is common. In rocks quartz often appears light gray to almost black because your see through it to the dark interior of the rock. See, for example, the quartz and pink orthoclase crystals in Bowen's Reaction Series. Quartz is often confused at first with Na Plagioclase, but plagioclase has cleavage faces and is translucent, not transparent.