Quartz is colorless and has low relief, and hence
is almost invisible under a single polarizing filter (left). Under
both filters, it has weak birefringence which
gives it a gray or white pattern. The crystal structure of quartz
is often deformed in rocks that have been under stress. The result
is often irregular, almost gradational boundaries between areas of that
show various shades of gray or white. When the microscope stage is
rotated, the gray patches seem to travel around like waves, producing what
is called wavy extinction.
The photos show quartz throughout the field of view except for some
small grains of biotite (the largest is at lower
left) and muscovite (at upper right).