The Japanese Katana is a sword that was once used by Samurai warriors in Japan’s feudal period. It was not only a weapon but also a work of art and a symbol of the Samurai’s code of honor, Bushido.
Its unique shape and beauty was the result of a highly developed forging technique that sought three much sought after qualities; the ability not to break or bend, a sharp cutting edge and a beautiful Hamon (blade pattern). This was achieved by the forging method known as Tatara-buki, which relied on black iron sand found in Japan, and which achieves rapid quenching at a low temperature, giving rise to a martensite steel, which is harder than brittle tempered steel. The hamon is where the transition of these two types of steel occurs, and variations in its shape, colour and beauty are an indication of the period, swordsmith or school of sword manufacture.
During the Kamakura era, sword craftsmen began to use a new technique called ‘Shinogizukuri’ in which the cross section of the body of the blade is rhombic instead of the earlier Hirazukuri and Seppazukuri. This made the sword stronger and easier to cut with. It is also at this time that the Kissaki, or tip, of the blade became rounded to make it more practical as a sword to use.
Today, the katana is no longer used as a weapon but as a ceremonial sword in some schools of Iaido and as decent art objects. As such it can be possessed without restriction in Japan. More information here