Please examine these figures to be sure that you recognize the positions of the melatope, isogyres, and isochromes.  You should also be able to predict the  principle vibration directions and birefringence throughout the figure, and determine the optic sign of the mineral.
 
 

Calcite crystal, optic axis interference figure	Calcite, with accessory plate (Z to NE)
Quartz crystal, optic axis interference figure	Quartz, with accessory plate (Z to NE)
Quartz crystal, rhombohedral section interference figure	Quartz, with accessory plate (Z to NE)
Quartz crystal (thick), optic axis inclined 15 degrees NE	Quartz crystal (thick), optic axis inclined 30 degrees NE