Steel plates with high hardness and high ductility are used. Brass symbolized hardness, strength, and firmness. Boron can also increase the surface hardness of steels and alloys through boriding. These metals share qualities of extreme hardness and corrosion resistance. Several boron compounds are known for their extreme hardness and toughness. Ice quality is judged by smoothness, friction, hardness, and brittleness. Aluminium oxide is used for its hardness and strength. The hardness and density of tungsten are applied in obtaining heavy metal alloys. Steels that can be tempered to great hardness. Polyurethane formulations cover an extremely wide range of stiffness, hardness, and densities. Oak wood has a density of about creating great strength and hardness. Teeth are made of multiple tissues of varying density and hardness. The Rockwell scale is a hardness scale based on indentation hardness of a material. Plastics vary immensely in heat tolerance, hardness, and resiliency. Often the carbide has a hardness of 1500 Vickers hardness.