- a quality of water that contains dissolved mineral salts that prevent soap from lathering
"the costs of reducing hardness depend on the relative amounts of calcium and magnesium compounds that are present"
- The hardness of diamond is 10.
- One of the qualities of steel is hardness.
- The hardness of diamond is such that it can cut glass.
- The hardness of God is kinder than the softness of men, and His compulsion is our liberation.
- Binder hardness
- For hardness tests
- high hardness alloys
- Rubber, hardness 60 IRHD
- Hardness: 50 Shore
- Hardness 50-60 Shore A
- Indications on water hardness (1 point)
- Hardness of metal cylinders and metal liners
- (depending on water hardness classes) 
- ISO 6506-1981 Metallic Materials — Hardness test — Brinell Test;
- Non-electronic hardness testing machines and appliances for metals