Stone is polished by hours of work by a skilled craftsman. There is no "coating" that is applied. The beautiful, nearly permanent shine on memorials is created by a series of diamond impregnated pads, water, and a skilled craftsman. If the polishing process is done incorrectly, the stone can overheat causing the stone surface to prematurely crumble.
There are times when cost is not the most important factor in a decision. When you only get one chance to get it right, a professional can make all the difference.
Memorialization is about healing the people that few people that mourn your loss and recording a little bit of the flavor of your character for the many people who will follow after you.