As I understand the Damascus process as used in rifle and shotgun barrels (and should be for knives as well), iron and steel is used in layers - hammer forged together. This overlapping and hammer 'welding' of the pre-twisted layers of steel and iron causes the different patterns to occur. Simply welded on strips, not twisted, resulted in plain, or Skelp twist, I think was the name - or perhaps 'Stub' twist English pattern designs. Picked up horse hoof nails from the cobble stone streets in London, were sold to gun barrel makers and were used in the process of barrel making.

More steel layers added to the iron layers increased strength - thus there were 2 layer and 3 layer steel/iron barrels.

After finishing, the black or dark is the iron layer, as the softer metal of the iron takes more of the colour of the finish (bluing or browning or simple oxidation), while the harder steel remains bright or takes on a softer hue of colour. If hard polished or burnished after finishing, the steel layers will be brightened, but the softer iron layer, which took the blue or browning finish more deeply, will remain dark. Burnishing may wear away the softer iron layers - or not - don't know - doubt it.

The outside should be smooth, unless it was as described by the Indian Artisan - phony, due to some sort of acidic etching process.

In my most humble opinion, of course.