If you want to make a far-away look of longing come into a medieval sword enthusiast's eyes, just mention Damascus steel. Originally referring to a kind of steel made from ingots of Wootz steel that came from India over two thousand years ago and was manufactured or traded in Damascus, it now refers to a whole class of steel marked by sinuous, wavy, light and dark banding patterns that resemble flowing water.
Because Wootz steel is no longer available, making true Damascus steel is now a lost art, but not for the want of many scientists and craftsmen trying to reverse-engineer the existing examples. However, the basic idea behind it is very well understood and if you go to a modern Renaissance faire you're likely to find many reproduction blades of surprisingly high quality for sale at the swordsmith's booth.
A Damascus steel blade is made by taking bands of iron and steel, heating them to red hot, and twisting them together. Then the smith hammers them out, reheats, retwists, and rehammers until the intricate, flowing pattern emerges. The result is a worked steel that the smith can control the properties of by controlling the carbon content, creating a tough, flexible steel for a sword's core and then welding on another steel that's been worked to be stiff and hard and can be sharpened to form the blade edges.