Hada & Hamon
The hada is the woodgrain like pattern formed in the surface of the steel. It is a result of the fold-forging process. A common myth of Japanese swords is that they are folded over a thousand times. In actual fact, they ae only folded between 10-15 times depending on the smith or school. However, with every fold the layers of steel increase exponentially.
The hamon is the crystalline pattern of the hardened cutting edge of the blade. This pattern is a result of the differential hardening process used to form an extremely hard cutting edge, while at the same time retaining ductile qualities in the back of the blade. This acts as a shock absorber: allowing the blade to flex slightly on impact.