When we observe different colourless and transparent mineral grains, some stand out more than others, although their thickness may be very similar.
This is the phenomenon known as relief. In the outline of the minerals, there are some darker lines which, according to their width, make the grains stand out to differing degrees.
Relief is an optical property that describes how well a material can be seen and distinguished from its surroundings.
It is obvious that grains 5 and 4 look better than 2 and 1. The relief goes up from grains 00 ==> 0 ==> 1 ==> 2 ==> 3 ==> 4 ==> 5.
We speak in these cases of relief: null, low, moderate, strong and extreme.