The Burr is CNC stainless steel, which has a high degree of material hardness (rating of HRC 55-58). The alloy also contains higher percentage of nickel and chromium which makes it anti-corrosive and rust free. CNC stainless steel is among the hardest to produce and most expensive, however the material properties make it ideal for a burr.
Stainless steel also has anti-static properties, which ensures that grind does not stick to the burr.