Skip to main content

Mohs' Scale

Mohs' scale

Mohs' hardness scale provides an index and relative measure of mineral hardness (i.e., resistance to abrasion). German geologist Frierich Mohs (17731839) devised a scale with specimen minerals that offered comparison of "hardness" qualities that allows the assignment of a Mohs hardness number to a mineral. Mohs' scale utilizes 10 specific representative materials that are arranged numerically from the softest (1) to the hardest (10). The reference minerals are (1) talc, (2) gypsum , (3) calcite, (4) fluorite, (5) apatite, (6) orthoclase feldspar , (7) quartz , (8) topaz, (9) corundum, and (10) diamond .

The softest mineral, talc, can be used in body powder. The hardest, diamond, is used in drill bits to cut through the most dense crustal materials. Mohs' scale is a relative index scale, meaning that a determination of Mohs'hardness number for a mineral is based upon scratch tests. For example, gypsum (Mohs' hardness number 2) will scratch talc (Mohs' hardness number 1). Talc, however, will not scratch gypsum. Glass is assigned a Mohs hardness number of 5.5 because it will scratch apatite (Mohs' hardness number 5) but will not scratch orthoclase feldspar (Mohs' hardness number 6).

Scratch tests are a common method used to identify mineral hardness relative to Mohs' scale. Streak tests are often carried out on streak plates. Mineral hardness is a fundamental property of minerals and can be used to identify unknown minerals. In the absence of comparative minerals, geologists often resort to common objects with a relatively well-established Mohs' hardness number. In addition to glass (5.5), copper pennies measure 3.5, and the average human fingernail averages a Mohs' hardness of 2.5.

The Mohs' scale is a comparative index rather than a linear scale. In fact, Mohs' scale has a near logarithmic relationship to absolute hardness. At the lower, softer end of the scale, the difference in hardness is close to linear, but at the extremes of harness, there are much greater increases in absolute hardness (e.g., a greater increase in the hardness between corundum and diamond than between quartz and topaz).

Hardness is a property of minerals derived from the nature and strength of chemical bonds in and between crystals . The number of atoms and the spatial density of bonds also influences mineral hardness. Softer minerals are held together by weak van der Waals bonds. The hardest minerals tend to be composed of dense arrays of atoms covalently bonded together.

Hardness characteristicsespecially in calcite crystalsmay vary as a property dependent upon the direction of the scratch (i.e., able show evidence of a particular Mohs' number if scratched along one face or direction as opposed to a different hardness number if scratched in a different direction).

See also Chemical bonds and physical properties; Field methods in geology; Mineralogy

Cite this article
Pick a style below, and copy the text for your bibliography.

  • MLA
  • Chicago
  • APA

"Mohs' Scale." World of Earth Science. . 21 Jan. 2019 <>.

"Mohs' Scale." World of Earth Science. . (January 21, 2019).

"Mohs' Scale." World of Earth Science. . Retrieved January 21, 2019 from

Learn more about citation styles

Citation styles gives you the ability to cite reference entries and articles according to common styles from the Modern Language Association (MLA), The Chicago Manual of Style, and the American Psychological Association (APA).

Within the “Cite this article” tool, pick a style to see how all available information looks when formatted according to that style. Then, copy and paste the text into your bibliography or works cited list.

Because each style has its own formatting nuances that evolve over time and not all information is available for every reference entry or article, cannot guarantee each citation it generates. Therefore, it’s best to use citations as a starting point before checking the style against your school or publication’s requirements and the most-recent information available at these sites:

Modern Language Association

The Chicago Manual of Style

American Psychological Association

  • Most online reference entries and articles do not have page numbers. Therefore, that information is unavailable for most content. However, the date of retrieval is often important. Refer to each style’s convention regarding the best way to format page numbers and retrieval dates.
  • In addition to the MLA, Chicago, and APA styles, your school, university, publication, or institution may have its own requirements for citations. Therefore, be sure to refer to those guidelines when editing your bibliography or works cited list.