Jewelling in guns usually has nothing to do with using jewels as bearings like in a watch. I can't say that could not be done, but I certainly have never heard of it. Gun "jewelling" is what is sometimes called engine turning. The process involves putting a round rubber or plastic tip (like a pencil eraser) impregnated with an abrasive into a drill press and bringing the tip down on the metal, cutting shallow rings into the metal. By overlapping these rings, a nice pattern can be created. Jewelling was originally used on gun interiors to hold oil, but today is usually used on visible surfaces for decoration.
It usually has little or nothing to do with smoothness, and in fact can make an action rougher if used in the wrong place, but its presence can mean that a gunsmith has given a action special care and attention.
One drawback is that in some places, like rifle bolts or revolver hammers, any rubbing of the part will score or "smear" the shallow jewelling and the result (IMHO) will look worse than if there were no jewelling at all.
Jim