Steel that is used for true Nihonto is called Tamahagane. The term itself can be loosely translated as jewel steel or jade steel. Tamahagane is a high carbon steel ore made from a very specific type of Japanese iron sand known as Satetsu.
Currently there are only a few places in Japan where Tamahagane is made. The chunks in the picture (myyy precious!!) are made by the Nittoho Tatara in Oku-Izumo town in Shimane prefecture, Japan.
The impurities in the iron sand are still somewhat present in the ore. If you look carefully you'll notice a lot of different colors. A lovely sight isn't it?