The hardest steel commonly used for swords is generally considered to be high carbon steel, specifically tool steels such as T10, L6, or O1. These steels are known for their high carbon content and are often used in the production of high-quality, durable blades.

Among these tool steels, T10 is particularly renowned for its hardness and durability. It is a high-speed tool steel that can achieve a high Rockwell hardness, typically ranging from 60 to 64 HRC (Rockwell hardness scale). This hardness allows the blade to retain a sharp edge for longer periods and withstand significant impact without chipping or deforming.

Another steel known for its hardness is L6, also known as "Bainite" steel. L6 offers excellent toughness and hardness when heat treated properly. It has a high carbon content and can achieve high levels of hardness, making it suitable for blades that need to endure heavy-duty use.

It's worth noting that the hardness of a sword blade is just one factor to consider. Other characteristics, such as flexibility, durability, and sharpness retention, also play crucial roles in determining a sword's overall quality and performance. Different types of steel can be optimized for specific purposes, and the choice of steel for a sword ultimately depends on the intended use and the preferences of the swordsmith or sword manufacturer.