The chief use of gypsum is in the manufacture of plaster of paris, during which it is pulverized and heated to drive off part of the water of crystallization so that its composition corresponds to CaSO₄·½H₂O. This powder, when mixed with water and poured into a mold, heats and sets; that is, it hardens by taking up enough of the water to restore its original composition. Although thick, wide-spread beds of gypsum occur in other localities, probably most of the gypsum in Missouri has been secondarily formed, as from the reaction of sulphuric acid from oxidizing pyrite on calcite; and its quantity is limited to small crystals, veins, and crusts in or on other rocks. Gypsum may be an impurity in coal, and some beautiful crystals a few inches long have been found in weathering clay deposits. It therefore cannot be considered as a commercially valuable mineral of this state. Meteorites Meteorites, the rock-like specimens which have come to our earth as sparkling meteors in the sky, are perhaps the most prized specimens which the average collector hopes to find, and perhaps more specimens are mistaken for meteorites than for any other geological substance. Meteorites are rare and not easy to find; they are also not easy to determine. The iron variety is usually a heavy, roughly-pitted, brown, tough, metallic, nickel alloy of iron. Therefore, a positive chemical test for nickel is usually strongly suggestive of a meteoric origin, but confirmation almost requires that a surface be polished and etched with dilute acids to bring out typical and characteristic structures. [Illustration: The polished and acid-etched surface of an iron meteorite. Shows the Widmanstatten figures characteristic of iron meteorites. (Photo courtesy of American Museum of Natural History, New York).] The stony variety of meteorites usually contains a rock-forming mineral called olivine, beneath its pitted brown surface. In case of either variety, since special equipment is required for final testing and determination, it is recommended that this be done at a laboratory appropriately equipped. Gold Gold is not known to occur in Missouri, except for very small quantities which have been carried into the state with the glacial deposits in the north half. Miners have searched carefully, and geologists have studied Missouri rocks intently, comparing them with the gold veins of the western states, but they find no promise of a gold deposit in Missouri. We have been favored with other geological products, but it is a waste of time to search for gold in Missouri. Silver Silver has been recovered from ore in the Silver Mines area in Madison county and from the galena of southeastern Missouri. Except for occurrences within the igneous rock area and the lead mining regions, geologists do not expect to find additional silver ore deposits.