Major forms of hamon on katana sword
In swordsmithing, hamon (刃文 hamon) (from Japanese, literally "blade pattern") is a result of the differential hardening process (quenching and tempering) on blade. The hamon is the outline of the hardened zone (yakiba) which contains the cutting edge (ha). Suguha (straight) and Midare (wavy) are two main classifications of hamon. There are many types of temper line (hamon) and quite commonly mixed styles such as choji-midare or midare-togari. Some of the major forms are shown below.
This tempering when done properly is not just visible on the surface, but goes all the way through the meat of the blade.