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shoulder. Medicines may incorporate sulfur or mercury or compounds extracted fromdolo- mite (milk of magnesia), epsomite (Epsom salts), bismuthite (Pepto-Bismol) or coal. The lead apron used to protect your gonads from mutagenic X-rays probably came from the ore galena. The X-rays were generated using tungsten (from wolframite) or thulium (from fluorite). Platinum, radium, and cobalt are used in chemotherapy for cancer. Barium shows up well in X-rays so it is often injected into the digestive system to help doctors visu- alize and diagnose medical problems such as cancer or bowel disorders. Granite, sandstone, limestone, marble, and gneiss are the most frequently used building stones. Granites are relatively resistant to weathering, and their low iron content keeps them from becoming stained with rust. Many building stones are pol- ished for decorative use in floors, tiles, foun- dations, walls, and counters. Limestone and marble building blocks often contain such spectacular fossils that some museums and universities conduct field trips along city streets to study the geology and paleontol- ogy revealed in the building stones! The brownstone buildings common in many of America’s eastern cities are made out of 230 million-year-old Triassic sand- stones, which are easily carved. The iron

alloys include vanadium, manganese, molyb- denum, and tungsten. Have you recently had medical attention? The surgeon’s cutting instruments may have been made from the metal tantalum (from tantalite ore). Perhaps an orthopedic surgeon fastened your bones together using a titanium screw like the one used to repair my frequently dislocated

Occasionally gypsum’s tabular crystals form in a roselike cluster called a desert rose. Gypsum also occurs in huge deposits up to ten meters thick which precipitated out of seawater as ancient seas evaporated. Gypsum is an important ingredient of paint, cement, and plaster.

For as long as humans have

existed, we have used rocks that break with a hard,

sharp, cutting edge to make

stone tools. This volcanic rhyolite was fashioned into a hand ax over a million years ago. Obsidian, quartz, flint, and chert were the most commonly used minerals by prehistoric tool- makers.

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