6. Orthoclase, white cleavable variety.
7. Quartz, transparent.
8. Topaz, transparent.
9. Sapphire, cleavable variety.
10. Diamond.
It seldom happens in determining the hardness of a mineral that its hardness exactly conforms to that of some one member of the scale. In such cases we generally estimate the hardness. For example, suppose a mineral was harder than 4, but softer than 5, and that it was nearer 5 than 4, then we would call its hardness 4-3/4.
In order to preserve the scale some operators use a three-cornered file, first cutting the mineral and then the scale until a number is found, which is abraded to about the same depth as the mineral under examination.
Since a set of minerals forming a scale of hardness is not always at hand, the following scale given by Chapman is appended:—
1. Yields easily to the nail.
2. Yields with difficulty to the nail or just receives an impression from it. Does not scratch a copper coin.