Figure 53 (a) A well-formed prismatic crystal of quartz (longest crystal length is 6 cm). (b) Plane-polarised light image of a quartz crystal in a silica-rich lava. The quartz is the well-formed crystal with low relief and no pleochroism in the centre of the image (field of view 7.5 mm across). (c) The same field of view as in (b) between crossed polars. Quartz has distinctive first-order grey interference colours and, when deformed (or strained) slightly, exhibits characteristic undulose (wavy) extinction, whereby not all of the crystal goes into extinction at the same time.