Quartz

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).
 

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