Note: 'Group' refers to type of optical properites. First observe:

• Is the crystal always dark under crossed polars ?

  If yes - isotropic; If no - anisotropic (GROUP 1)

• If light passes through the crystal under cross polars, can one (uniaxial) or two (biaxial) orientations be found where the crystal appears dark? (GROUP 2)

Shapes of unit cells are determined by the crystal system, with the following results:

CRYSTAL SYSTEM	AXES LENGTHS	AXIAL ANGLES
CUBIC	All equal	All 90
TETRAGONAL	a=b not = c	all 90
HEXAGONAL	two are equal	2 are 90, one 120
ORTHORHOMBIC	all unequal	all are 90
MONOCLINIC	all unequal	two are 90
TRICLINIC	all unequal	none are 90

These are the detailed shapes associated with these crystal systems!

Example showing how optical properties that provide information about the crystal system can help distinguish similar looking gems!:

Blue sapphire - hexagonal - thus uniaxial.

Blue colored cubic zirconia (looks like sapphire) - cubic thus isotropic

Tanzanite (blue like sapphire) - orthorhombic thus biaxial.