English: Katana - Showing alternating layers of hardenability. In this photo, lighter colored layers have a higher carbon content than the darker layers. In reality, which layer is lighter or darker depends on the angle it is viewed from compared to the lighting.

Visible on the right hand side of the photo is the nioi, which can be seen as a bright thin line about a millimeter or two wide surrounding the hardened edge of the yakiba. The nioi is made up of niye, which are single martinsite crystals surrounded by pearlite. The streaks of niye that whisp away from the hardened edge result from the forged in layeres of different steel.^[1]

1. ↑ "A History of Metallography" by Cyril Smith