What is Ayasugi Hada in Japanese Katana terminology?

"Ayasugi Hada" or 綾杉肌 in Japanese, refers to a pattern on the surface of the ground iron (jigane) that resembles the grain of Ayasugi, a type of cedar wood. This pattern appears like large waves or ripples. The Ayasugi Hada is characteristic of the Oshu Gassan school, hence it is also referred to as "Gassan Hada". However, this pattern is also commonly seen in the works of the Satsuma Naminohira school.