That is Damascus. It was used in swords and knives as a way to make a high quality steel by hand. Gun makers used the method a little more than 100 years ago to make barrels, usually for a shotgun, by wrapping a strip of steel in a spiral and forging the seams together.
Now the raw material is usually created by machine. It comes in different patterns and is pretty expensive. I think they have come up with some finishes to try and replicate it but all I've seen are poor imitations.
By the way, those grips are incredible.