How can diamond have a fixed refractive index? I was doing a lesson on solid states. There were given two kinds of solids namely amorphous solids and crystalline solids. Some differences between these two solids were given in my book. One of the difference was that crystalline solids are anisotropic in nature whereas amorphous solids are isotropic in nature.
Following is from my book

Crystalline solids are anisotropic in nature, that is, some of their physical properties like electrical resistance or refractive index show different values when measured along different directions in the same crystals.

Diamond is a crystalline solid. That means it is anisotropic in nature. This means that measuring the refractive index of diamond from different direction will give different values of refractive index. Then why we have fixed value of refractive index of diamond.
Not just about diamonds, many other crystalline solids have fixed refractive index.
Your textbook is wrong. Some crystalline solids are optically anisotropic. Many crystalline solids are indeed optically isotropic in nature. In solids like this the light beam experiences the same electron density regardless of the direction the crystal is oriented. Some examples of that are (obviously) diamond, halite and sylvite (sodium and potassium chlorides), garnets, spinels and more. Note that this is a feature derived from the electronic structure and it has nothing to do with the actual shape of the crystal.

Even in the case of anisotropic crystals, some are so weakly anisotropic they may considered isotropic in practice. Zeolites are a perfect example of those.