Mohs' scale of hardness, devised by the Austrian generations of geologists. It is a hardness scale for mineralogist F. Mohs in 1824, is familiar to many minerals based on the ability of a harder mineral to scratch a softer one. The scale is as follows:

It has been generally accepted that the scale is one of rank order only and that the steps between the various minerals are not equal. For example, one noted mineralogical textbook (B6rner 1962) says: 'The figures 1 to 10 on the scale merely denote an order of hardness, and have no quantitative significance, and the difference in hardness between successive grades is in fact very variable'. Another (Milner 1962) tells us: 'It is important to realize that Mohs' scale is quite arbitrary and the numbers do not represent any regular mathematical ratio as between one and another'. Indeed, the author himself has repeated this view (West 1981). The purpose of the present note is to present new experimental evidence that shows this belief may have to be reassessed.

During the course of recent research into the wear of tunnelling machine tools, an apparatus was made to determine quantitatively the abrasiveness of rocks. In the apparatus a sharp steel conical point of cone angle 90° is applied to the plane surface of a rock specimen under a total load of 70 N. The rock is then slowly displaced by 10 mm. The abrasiveness of the rock is obtained by measuring the diameter of the resulting wear

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