Quartz is an enantiomorphic crystal, i.e., both right-handed and left-handed crystals exist.  The crystal structures of the two forms are mirror images.  Neither can be made to look like the other by rotation.  Both kinds could be equally useful, however, resonator manufacturing processes have been standardized on right-handed quartz.
	Quartz has trigonal structure.  The ideal crystal is a hexagonal prism with six cap faces at each end.  The prism faces are called the m-faces, and the major cap faces are called the r-faces; they are also called the major rhomb faces. The z-faces are called the minor rhomb faces.
	The Z-axis is also called the “optic axis,” and the “c-axis”. It is an axis of three-fold symmetry, i.e., all the physical properties repeat every 120o as the crystal is rotated about the Z-axis. The polarization of a beam of plane polarized light passed through quartz along the Z-axis will be rotated by the crystal.  The polarization is rotated clockwise in right handed quartz, as seen by an observer looking through the quartz towards the light source.  It is rotated counterclockwise by left handed quartz.  This “optical rotation” ability is used in optical instruments.  (It has no significance in frequency control applications.)
	C. Frondel, The System of Mineralogy, Vol. III, “Silica Minerals”, John Wiley and Sons, Inc., New York, 1962.