The density of iridium is 22.56 g/mL, almost as dense as osmium. While meteorites tend to be rich in iridium, iridium is not found in large quantities on earth. However, there is a layer of earth called the K-T boundary which is rich in Ir. Below the K-T boundary, dinosaur fossils are found. Above the K-T boundary, no dinosaur fossils are found. These observations led to the theory that a meteor strike caused the extinction of the dinosaurs. A cylinder of iridium metal is used as the prototype kilogram for the International System of Units.