The hamon is there, lying within the reach of the knifemaker who is willing to take the time to thoroughly rub the steel to enhance the elusive line.
The hamon (pronounced “huh-mown”) is a Japanese invention, in popular culture most widely recognized on the traditional Japanese Katana. The development of the hamon, along with the Katana itself is attributed in legend to the swordsmith Amakuni Yasutsuna, around 700 AD.
The hamon is a visual demarcation, showing up when etched as a wavy line across the surface of the steel. When etched, the acid eats away more at the softer section, and so shows up darker, and the harder edge section, showing up lighter, sometimes with a dark band where the two meet. This should not be confused with a “temper line”.
The hamon is caused by differential hardening (Yaki ire); the cutting edge, is hardened steel (martensite), while the spine is kept soft and tough (pearlite, ferrite, bainite, cementite), and so less prone to breakage.
This difference in hardness is the objective of the process; the appearance is purely a side effect. However, the aesthetic qualities of the hamon are quite valuable, not only as proof of the differential-hardening treatment but also in its artistic value where the patterns can be quite complex.