The higher the purity of ice (pure ice is harder than impure ice at the same temperature), the higher the hardness. The lower the ambient temperature, the higher the hardness. At MINUS 50 degrees, the hardness of ice can reach 6, which is higher than iron and close to glass. It is also possible to achieve a Mohs hardness of 7, which is similar to the hardness of the hardest granite.
Ice is changed from water. The molecules change regularly. The lower the temperature, the greater the hardness. The solid form of water molecules is easily influenced by external kinetic energy, which leads to molecular movement and improved toughness. In addition, low temperature affects the molecular motion by n times. And even if there are cracks, you can slowly repair (sublimate) yourself. The second is to become extremely regular, but the structure is not as stable and fragile as carbon atoms.