Plural of hardness; multiple instances or types of the quality of being hard, firm, or difficult.
Plural form of 'hardness' (from 'hard' plus '-ness'). The plural is rare in modern English since 'hardness' is usually abstract and uncountable, but it appears in technical contexts describing multiple measurements or varieties.
In geology and materials science, 'hardnesses' becomes countable when scientists talk about the hardness of different minerals or materials using scales like Mohs hardness—turning an abstract quality into measurable data points.
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