 Uranium occurs naturally in low concentrations of a few parts per million in soil, rock, and water, all over the world. Over 150 uranium-bearing minerals have been identified out of about 5,400 minerals recognized by the International Mineralogical Association. Of all of these, zircon is the most significant. A zircon mineral grain forms when it first cools from its liquid temperature to a solid, much like liquid water at its freezing point. This is called its trapping temperature because it traps elements like uranium and pushes out elements like lead. This makes it excellent for uranium-dating purposes. It effectively sets the uranium-led clock to zero. Lead atoms created by uranium decay are tracked in the crystal and build up in concentrations over time. Producing high precision ratios of uranium to lead in a zircon crystal requires multiple steps, including isotope dilution, thermal ionization, and mass spectrometry, which we have just covered. And to start with, there is a procedure called chemical abrasion to isolate the best zircon crystals in the rock. Here's how the Memorial University of Newfoundland does it. First of course, they find all looking rocks. The geology associated with this step is a science in its own right. 20 kilogram rock samples are typical. That's 44 pounds. Their first step is to wash and crush the rock into chip-sized pieces. They pulverize the chips into powder and pan the powder to select only the densest particles. At this point, we're now down to around a few hundred grams of material. That's just seven ounces out of the original 44 pounds. Then they further separate the minerals by density through a heavy liquid. The densest materials will reach the bottom first. This takes advantage of zircon's extreme density over most other minerals. At this point, they have a few grams of zircon with a few other minerals. They then pass the remaining material through a magnetic field to separate out the grains with the most iron. At this point, they have a few milligrams of zircon with all other minerals removed. Using jeweler's tweezers under a microscope, they manually select the very best 40 or 50 zircon crystals. They put these under an electron microscope. This brings out the inner structure of each crystal. Some are damaged and discarded. Some are pristine and used. Each crystal is cleaned and put into a vial of hydrofluoric acid and baked for three days until dissolved. All zircon molecular content is now in liquid form, including silicon, zirconium, uranium, and lead. Then they use resins and acid washes to remove all elements except uranium and lead. We're left with a single drop of water that contains all and only uranium and lead in the original zircon crystal, usually around 10 to 100 picograms. A picogram is a trillionth of a gram. This is the chemical abrasion process. The result is called the analyte, the substance to be analyzed. They then add a carefully measured amount of lead 205 to the analyte. This is the isotope dilution step. The analyte is then placed on the rhenium thermal ionization strip where it is rapidly converted from a drop of liquid to a gas. In the process, all the atoms have electrons stripped, making them positively charged ions. This is the thermal ionization step. The now charged ions flow into the mass spectrometer. Over hours, the mass spectrometer reports the counts and ratios for each stream. This is the mass spectrometer step. These ratios feed the equations that tell us the age of the rock they came from. This rock was 410 million years old.