Birefringence

Anisotropic minerals have a range of Refractive index values at different directions. Out of these values, the Maximum RI value and the minimum RI value can be observed. The difference between the maximum and minimum values is defined as "Birefringence". The interference colours shown by a mineral depends on its birefringence. Thus based on the birefringence, all interference colours are put into a scale known as "Newton's scale of interference colours".
Anisotropic minerals are classified according to their birefringence values as,
0-0.018 - low birefringence - gives colours of high order in the scale
0.018-0.036 - moderate birefringence
0.036-0.055 - high birefringence
0.055< - very high birefringence - gives colours of high order in the scale.

birefringence chart

Note that as birefringence increases, the colours in the chart repeat but the shades become pale.

It should also be noted that when the thickness of the section of the same mineral is increased, its birefringence also increases producing the above colour scale. Therefore the above scale can also be produced with a single mineral of varying section thickness.