For biaxial minerals optic sign is dependant on whether the X or Z indicatrix axis is the acute bisectrix.

• if Bxa is X, mineral is -ve
• if Bxa is Z, mineral is +ve

In the special case where 2V = 90°, mineral is optically neutral.

Another convention used is to identify the angle between the optic axes bisected by the X axis as the 2V_X angle; and the Z axis as 2V_Z angle.

These two angles can vary from 0 to 180°, such that the following relationship holds:

Using this convention the optic sign is determined by the following:

• if 2V_Z < 90°, the mineral is +ve.
• if 2V_Z > 90°, the mineral is -ve.

Light travelling through biaxial minerals is split into two rays - FAST and SLOW rays which vibrate at 90° to each other.

The vibration directions of the FAST and SLOW rays are defined, or determined, by the axes of the ellipse or section through the indicatrix, which is oriented at 90° to the wave normal.

The Refractive Index corresponding to the FAST ray will be between n_alpha and n_beta, and is referred to as n_alpha'.

The Refractive Index corresponding to the SLOW ray will be between n_beta and & n_gamma, and is referred to as n_gamma'.

With this convention the following relationship will be true for all biaxial minerals:

1. X - will always correspond to the fast ray and will have the lowest RI.
   
   • RI = n_alpha, always fast
2. Y - will be either the fast or the slow ray depending on which other indicatrix axis it is withand its refractive index will be between the lowest and highest RI for the mineral.
   
   • RI = nbeta, either fast or slow
3. Z - will always correspond to the slow ray and will have the highest RI.
   
   • RI = n_gamma, always slow.