Robby Keating, age 13, of Las Vegas, Nev., for his question:

CAN YOU EXPLAIN THE SCALE OF HARDNESS?

Hardness is the ability of a material to scratch a mark on other substances, or to resist being scratched by them. Scientists measure the hardness of a material by comparing it with 10 well known materials and they call the list the standard scale of hardness.

The minerals are arranged in order from 1 to 10. Each mineral in the table scratches the ones with lower numbers and can be scratched by all those with higher numbers.

Here is the standard scale of hardness: 1. Talc; 2 Gypsum; 3. Calcite; 4. Fluorite; 5. Apatite; 6. Feldspar; 7. Quartz; 8. Topaz;

9. Corundum; 10 Diamond.

To test another substance, you match it against the minerals of the hardness scale. You can get an approximate idea of the hardness of a mineral by using your fingernail, a copper coin, a knife blade or a piece of window glass.

When minerals must be accurately tested, as in the manufacture of tools and gears, machinists use an instrument called a sclerometer. This device registers the force required to dent or scratch the material with a diamond or borazon, the hardest substances known.