Believe it or not, Diamond is not the most expensive useable material found on Earth. Synthesized in 1950 by Lawrence Berkeley National Laboratory, Californium is considered the most expensive material known to mankind. Created by the bombarding of curium with alpha particles, Californium is a highly radioactive material that is often not found in biological environments, however, traces of Californium has been found in the soil of Earth.
The reason for Californium being so expensive is that the process needed to synthesis the material is a lengthy process of multiple fissions and the amount produced is in such small quantities. Every single year there is only a half gram of this material produced. This small quantity goes for about $25 million per gram. Californium is also only made by two laboratories in the entire world, showing how hard and dangerous it can be to make it.
So why would people pay so much for Californium? The reason people would purchase Californium is that it functions in portable transfer or neutrons and aids in the detection of gold and silver ores and detecting metal fatigue and stress in airplanes.
Image Credit: U.S. Department of Energy