☙

Mohs’ scale

A scale of hardness primarily used for minerals, devised around 1812 by Friedrich Mohs (1773 — 1839), a German minerologist, and published by him in 1824.¹

Mohs’ scale consists of 10 common minerals in order of increasing hardness (shown in the second column below). An unknown mineral's place on the scale is determined by what it will scratch and what will scratch it. For example, the average fingernail will scratch gypsum and can be scratched by calcite, so fingernail would be given a rating between 2 (gypsum) and 3 (calcite). Window glass is about 5.5, which shows ordinary quartz (7) will scratch it.

Although it is very convenient, there are many problems with using scratching as a test for hardness. For example, many minerals scratch more easily in one direction than another, due to their crystal structure.

Increasingly sophisticated methods for measuring hardness showed that the steps in Mohs' scale are far from equal, especially at the high end. For example, on an equal-step scale, if diamond is taken as 10, corundum would be 2.5 and topaz 1.6. Taking advantage of very hard synthetic materials that weren't available to Mohs, the top end of the scale has been revised² to provide finer (but not equal) gradations, as shown below.

	Original scale	Revised scale
1	talc	talc
2	gypsum	gypsum
3	calcite	calcite
4	fluorite	fluorite
5	apatite	apatite
6	orthoclase	orthoclase
7	quartz	vitreous pure silica
8	topaz	quartz
9	corundum	topaz
10	diamond	garnet
11		fused zirconium oxide
12		fused alumina
13		silicon carbide
14		boron carbide
15		diamond

1. F. Mohs.
Grundriss der mineralogie.
1824.

2. R.R. Ridgeway, A.H. Ballard and B.L. Bailey.
“”
Transactions of the Electrochemical Society. volume 63, page 267 (1933).