One of the sapphires, just 0.9 millimeters across, from the Volcanic Eifel. (Sebastian Schmidt)
ScienceAlert has an article that discusses the origin of sapphires. For many years, scientists have wondered about the origin of sapphires. Does their origin involve volcanism or something else? Sapphires are made of predominantly of aluminum oxide in the form of corundum. The various colors come from mineral impurities. Scientists looked a 223 specimens of sapphire from volcanic deposits near Eifel, which is located between Germany, Belgium, and Luxembourg. They were able to determine that the sapphires formed at the same time as the volcanism. The research was pubiished in the journal Contributions to Mineralogy and Petrology.
The deep blue hue that sapphires are known for comes from titanium and iron tinting the corundum, for example. Iron on its own makes yellow sapphires, and can also give us green stones. Chromium turns the corundum pink or red, and that's how we get rubies.
What's more, whole other minerals – such as rutile (titanium dioxide) and zircon – can get trapped within sapphires as they form.
Scientists can then use these minerals to determine when the crystal bloomed. That's because as these rutile and zircon form they incorporate uranium, which then undergoes radioactive decay at a known rate. Scientists can study the ratios of uranium to lead inside the rocks to determine how long that uranium has been decaying.
In addition to the uranium, he researchers studied the sapphires' oxygen isotope ratios. An isotope is a form of an atom with a different number of neutrons, and there were two isotopes relevant to the study. Oxygen 16, with 8 protons and 8 neutrons, is the lighter isotope, and the most abundant form of oxygen on Earth. Heavier oxygen-18 has 8 protons and 10 neutrons, and is more abundant in minerals from the deep crust than in minerals from the mantle.
By studying the ratios of these isotopes, the researchers were able to determine that the Eifel sapphires had oxygen ratios that could be traced both to the mantle and to the crust.
