In diamond, you can embed a “spy.” Simply replace a carbon atom with nitrogen: you get a defect that responds to magnetic fields and sends a light signal.
But ordinary spies can’t withstand colossal pressure. Scientists invented a tempering process: after nitrogen implantation, the diamond is annealed at extreme temperature and pressure. This strengthens the defects, letting them work even at 240 gigapascals, such pressure that diamond itself would flow if not prepared. Curiously, the spies sit right inside the diamond anvil that generates this pressure.
Previously, studying magnetic properties of materials under such pressure was impossible. Now, the spies will help reveal the secrets of superconductors that work without cooling. For instance, hydrogen at high pressure can become a perfect conductor, but its magnetic mysteries are yet to be spied on.
240 gigapascals is the pressure of a freight train balancing on a pinky fingernail. Even diamond needs tempering to survive such a squeeze.