Rhonda Ballou, age 11, of Ord, Nebraska, for her question:
Which is the hardest kind of ordinarv rock?
The hardest natural mineral is the diamond. But it is as scarce as hen's teeth, so we hardly can rate it as an ordinary rock. Nor can we count precious topaz or the corundum gem stones. These various jewels are in classes eight, nine and ten on the mineral scale of hardness. The hardest of the ordinary everyday rocks is in class seven but in some ways it is superior to the rare precious gems.
It was nice of the earth to add so much beauty to the hardest ordinary rock and also to make so much of it. Its common name is quartz.
Chemically, quartz is a silicate mineral, which means that it is a compound of the very plentiful elements, silicon and oxygen. Its tough molecules were packaged in the molten magmas and lavas of fiery volcanos. The mixture cooled and froze to form igneous rocks. Quartz may be mixed with other minerals in rocks such as granite and feldspar. Sometimes a massive layer of solid milky white quartz hides secret veins of gold. Other common quartz rocks are pink or green, blue or brown or almost black. Milky white quartz forms those smooth beach pebbles that look like ostrich eggs.
A lot of common quartz rock has been transformed by geological happenings such as heat, pressure and moving water. Some quartz lavas cooled so fast in the chilly air that they formed black volcanic glass called obsidian. Other quartz molecules had time to arrange themselves in clear glassy crystals, shaped in six sided columns with six sided pointed peaks. If traces of manganese were present, the crystals were tinted with rosy pink or violet amethyst. Other impurities tinted them smokey grey or brown. Fine fibers of asbestos added a shimmering glow to create cat's eyes and tiger's eyes.
Some quartz beds were steeped for ages in seeping ground water. Their molecules were washed away and deposited elsewhere in layers of semi precious stones. Often they lined the pockets of porous rocks with waxy chalcedony. Its translucent colors range from black through moonbeam blues, grays or white. Various impurities changed other deposits to red yellow carnelians, apple preen chrysoprace or to onyx, striped with ribbons of rich browns and white.
The most handsome semi precious quartz stones are speckled or striped agates. And sometimes the seeping quartz added moisture to create precious opals.
Naturally the earth's hardest ordinary mineral is very useful. Sand for making glass and concrete is mostly quartz. Clear quartz crystal, or rock crystal, is used to make superior lenses, lab equipment and radio crystals. Our remote ancestors found hard nodules of blue black flint embedded in limestone. They shipped and flaked them to shape some of mankind's very first tools.