Hard water
Water's hardness is determined by the concentration of multivalent cations in the water. Multivalent cations are positively charged metal complexes with a charge greater than 1+. Usually, the cations have the charge of 2+. Common cations found in hard water include Ca and Mg. These ions enter a water supply by leaching from minerals within an aquifer. Common calcium-containing minerals are calcite and gypsum. A common magnesium mineral is dolomite (which also contains calcium). Rainwater and distilled water are soft, because they contain few ions.
List of blade materials
The hardness of steel is usually stated as a number on the Rockwell C scale (HRC). The Rockwell scale is a hardness scale based on the resistance to indentation of a material, as opposed to other scales such as the Mohs scale (scratch resistance) testing used in mineralogy. As hardness increases, the blade becomes capable of taking and holding a better edge, but is more difficult to sharpen and more brittle (commonly called less "tough"). Laminating a softer steel between a harder one is an expensive process that to some extent gives the benefits of both types (see Damascus steel).