It is the same as electroplating. The coating is very thin.
Just to add a bit more info on finishing internal parts. Jeweling is done to hold lubricant and it simply looks wonderful.
Whenever I see the internals jeweled or gold plated, I think of a fine man's definition of a man of conscience. It is what one does when no one is looking.
This embellishment is almost never seen but by a gunsmith. Somewhat a testimonial of a maker that did his very best even though no man might see it.
Best,
John

Humble member of the League of Extraodinary Gentlemen (LEG). Joined 14 March, 2006. Member #1.