Tim Driggers, age 12, of Lexington, S.C., for his questions

What is the hardest natural substance?

The minerals of the earth are rated according to a scale of hardness. There are ten classes in this scale and class one is the softest of minerals. Talc in class one, is so soft that it can be scratched by any other mineral. Gypsum in class two can scratch talc, but it is not hard enough to scratch calcite in class three. Fluorite, in class four, can scratch calcite, but it cannot scratch apatite in class five.

Orthoclase in class six cannot scratch quartz in class seven and quartz cannot scratch topaz in class eight. Rubies are in class nine. There are, of course, many minerals in each of these nine classes. But class teno the hardest, has but one mineral, the diamond. The diamond is the hardest natural substance. It can scratch any other stone and no stone can scratch it back.