Gemstones
Johannes,
Not all "gemstones" are the same. You might want to red this explanation of What makes a gemstone." Fact is, olivine weathers easily and rapidly when exposed to water. Its just a fact of life. This doesn't mean it just melts away when you accidently wash peridot costume jewelry. But when exposed to wet conditions over time, olivine decomposes more rapidly than other "gemstones." Based upon this fact, scientists can use its presence on the surface of Mars to make reasonably accurate explanations about how wet the martian climate may have been billions of years ago. Donald