Comparing Hardness of Rocks

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.

Is it static or current when you rub two rocks together?

          Static electricity happens when we pile electrons on the surface of an object and they can't get away. It can also happen if we rub electrons OFF an object. When electrons move, we call that current.

          When we rub two rocks together, if they are the right kind of rocks (quartz) and it is dark enough, we see sparks. These sparks are caused by a different effect, although it also has to do with electrons. The rock is made of crystals, and the atoms in the crystals are connected with very strong forces. When we crush or break the crystals by banging or rubbing the rocks together, we release some of that energy. The energy excites electrons in the atoms, they bounce around and as they relax back into place, they send out light. We see this light as sparks. This is called piezoelectricity.

          By the way, you can see the same thing in a very dark, like pitch black, room with Wint-O-Green Lifesavers as the sugar or flavor crystals break. (It's not polite to chew with your mouth open unless you are in the dark with Wint-0-Green Lifesavers. Then it is science!)