Diamonds are almost impossible to detect directly because they are so rare: very rich kimberlite pipes, the routes through which diamonds rise, may contain only three carats of diamonds per ton of kimberlite. kimberlite begins as magma in earth’s mantle (the layer between the crust and the core). as the magma smashes through layers of rock, it rips out debris, creating a mix of liquid and solid material. some of the solid material it brings up may come from a so-called diamond-stability field, where conditions of pressure and temperature are conducive to the formation of diamonds. if diamonds are to survive, though, they must shoot toward earth’s surface quickly. otherwise, they revert to graphite or burn. explorers seeking diamonds look for specks of “indicator minerals” peculiar to the mantle but carried up in greater quantities than diamonds and eroded out of kimberlite pipes into the surrounding land. the standard ones are garnets, chromites, and ilmenites. one can spend years searching for indicators and tracing them back to the pipes that are their source; however, 90 percent of kimberlite pipes found this way are barren of diamonds, and the rest are usually too sparse to mine. in the 1970’s the process of locating profitable pipes was refined by focusing on the subtle differences between the chemical signatures of indicator minerals found in diamond-rich pipes as opposed to those found in barren pipes. for example, g10 garnets, a type of garnet typically found in diamond-rich pipes, are lower in calcium and higher in chrome than garnets from barren pipes. geochemists john gurney showed that garnets with this composition were formed only in the diamond-stability field; more commonly found versions came from elsewhere in the mantle. gurney also found that though ilmenites did not form in the diamond-stability field, there was a link useful for prospectors: when the iron in ilmenite was highly oxidized, its source pipe rarely contained any diamonds. he reasoned that iron took on more or less oxygen in response to conditions in the kimberlitic magma itself—mainly in response to heat and the available oxygen. when iron became highly oxidized, so did diamonds; that is, they vaporized into carbon dioxide. the primary purpose of the passage is to(a) discuss an objection to gurney's theories about the uses of indicator minerals(b) explore the formation of diamonds and the reasons for their scarcity(c) analyze the importance of kimberlite pipes in the formation of diamonds(d) define the characteristics of indicator minerals under differing conditions(e) explain a method of determining whether kimberlite pipes are likely to contain diamonds