- Mohs hardness scale:
- talc (absolute hardness = 1): a fine grained mineral having a soft soapy feel and consisting of hydrated magnesium silicate; used in a variety of products including talcum powder.
- gypsum (absolute hardness = 3): a common white or colorless mineral (hydrated calcium sulphate) used to make cements and plasters; especially, plaster of Paris.
- calcite (absolute hardness = 9): a colorless or white crystalline mineral that is a form of calcium carbonate from limestone, marble, chalk and used in cement, plaster, glass, and paints.
- fluorite (absolute hardness = 21): a variously colored crystalline mineral consisting of calcium fluoride; a chief source of fluorine.
It is considered to be a beautiful transparent mineral found in many parts of the world; however, although pure fluorite is colorless, it often has trace elements in the mineral resulting in purple, blue, green, red, and yellow colors.
- apatite (absolute hardness = 48): a glassy, variously colored calcium phosphate mineral used in fertilizers and as a source of phosphorus.
- orthoclase (absolute hardness = 72): a variously colored feldspar, a common white or pink mineral having two good cleavages at right angles, and found in silica-rich igneous rocks; used in the manufacture of porcelain.
Feldspar consists of silicates of aluminum with potassium, sodium, calcium, and, rarely, barium. About 60 percent of the earth's outer crust is composed of feldspar.
- quartz (absolute hardness = 100): one of the most common minerals, silicon dioxide, having many varieties that differ in color, luster, etc., and occurring either in masses (as agate, bloodstone, chalcedony, jasper, etc.) or in crystals (as rock crystal, amethyst, citrine, etc.).
It is the chief constituent of sand and sandstone, and an important constituent of many other rocks. It is piezoelectric and used to control the frequencies of radio transmitters.
- topaz (absolute hardness = 400): a colorless, blue, yellow, brown, or pink aluminum silicate mineral, often found in association with granitic rocks and valued as a gemstone; especially, in the brown and pink varieties.
- corundum (absolute hardness = 400): a hard mineral form of alumina that crystallizes into a range of colors and is used as gems and abrasives.
- diamond (absolute hardness = 1600): a pure or nearly pure, extremely hard form of carbon, naturally crystallized in the isometric system used as a hard transparent precious stone that is used for gems, abrasives, and cutting tools.
The scale is not regular in the sequence of hardness in that diamond, at number 10, the hardest natural substance is four times harder in absolute terms than corundum, at number 9.
Here is a mnemonic that might help you remember the sequence of the Mohs scale: The Geologist Can Find An Ordinary Quartz, (that) Tourists Call Diamond!