why grain boundaries are visible?

Answer 1

boundaries between crystal grains are less well defined than the actual crystals, and have a different refractory index or optical density.

In anisotropic crystals, the refractory index or the surface structure of a transparent cut also depends on the orientation of the crystal.

Answer 2

The short answer is that anything "visible" emits photons for you to see. The crystaline structure at grain boundaries (orientation of atoms (or molecules) is different than across the grain and emission of photons is also different perhaps including polarization of light.