Polishing is the process of creating a smooth and shiny surface by rubbing it or using a chemical action, leaving a surface with a significant specular reflection (still limited by the index of refraction of the material according to the Fresnel equations.). In some materials (such as metals, glasses, black or transparent stones), polishing is also able to reduce diffuse reflection to minimal values. When an un-polished surface is magnified thousands of times, it usually looks like mountains and valleys. By repeated abrasion, those "mountains" are worn down until they are flat or just small "hills." The process of polishing with abrasives starts with coarse ones and graduates to fine ones.
The strength of polished products is normally higher than their rougher counterpart owing to the removal of stress concentrations present in the rough surface. They take the form of corners and other defects which magnify the local stress beyond the inherent strength of the material.
Polishing with very fine abrasive differs physically from coarser abrasion, in that material is removed on a molecular level, so that the rate is correlated to the boiling point rather than to the melting point of the material being polished
Above info is referenced from Wiki