Knoop indentation hardness numbers may be obtained on polished mineral grains as small as 10 microns in diameter. This paper gives average Knoop numbers for 127 opaque minerals and quartz. The limits of tests and the average hardness number are plotted on a logarithmic scale on which the lines representing the hardness range for a given mineral are reasonably comparable throughout the range from 1 to 1500. Some minerals exhibit striking differences in hardness in different crystallographic orientations.