Marie Shinskie, age 13, of Cedar Grove, W, Va., for her question:

Why does a diamond cut glass?

Most of the stones and rocks seem pretty hard to us. But some are harder than others and you can test this for yourself. Talc stone is so soft that you can cut into it with a sharp knife. The other common everyday stones and pebbl(:s range from harder than talc and softer than flint. The rocks of the earth have been arranged in a scale of hardness. There arc. ton classes in this scale and every natural stone has its number of hardness from one to ten.

Talc is rate one on the scale of hardness. Being the softest, it cannot scratch the stones in the other nine groups. Gypsum is rated two. It can scratch only minerals of the talc class. Calcite represents group three, fluorite four, apatite five, feldspar six, quartz seven, topaz eight and corundum nine. Each mineral can scratch all the stones in the group below it and they cannot scratch back. However, it cannot scratch the harder stones in the higher groups,

The only natural stone in group ten is the diamond. It is hard enough to scratch all the other stones and not one of them can scratch it back. For the diamond is the hardest natural substance in the world. It can cut glass because glass rates between five and seven on the scale of hardness, depending upon its quality.