We were asked how students can compare the hardness of different rocks, especially scoria and volcanic tuff.
To compare two different materials’
hardnesses, one approach is to try to scratch one with the other. We suspect,
although we haven’t tried, that you will find the scoria scratches the tuff and
not vice versa. This method is used by gemologists, who know that a rock that
can be scratched by a common steel nail will be too soft to take a high polish.
Steel is just about the perfect hardness to make this determination. Agate is
harder than steel and polishes to a high luster, limestone is softer and
remains dull no matter how long it is polished. A diamond should be able to scratch almost anything, and talc shouldn't scratch anything.
