I45mm Midnight Edition Rotary Blades! They are one of the first black blades in the market! Made out of carbon steel, they are designed with a universal opening so they will fit on other rotary cutters with the same opening. Material: They are made out of carbon steel, the same metal and process that we use to make our Midnight Edition Fabric Shears. Blade Durability: This can vary from person to person and how one uses their cutter, as well as the type of cutting mat they have. Blades were tested at various fabric shops, and after using them for up to 8 hours a day, five to seven days a week, each shop got between 3 - 4 months of use from one blade.