Diamond also has relatively high optical dispersion (ability to disperse light of different colors).
The conditions for diamond formation to happen in the lithospheric mantle occur at considerable depth corresponding to the requirements of temperature and pressure.
These depths are estimated between 140 and 190 kilometers (87 and 118 mi) though occasionally diamonds have crystallized at depths about 300 km (190 mi).
Sitting outside with my back to the sun, I noticed that the shadow cast by each clear lens of my glasses was equally as dark as that cast by the frame and my head. Surely the clear lens would let the light through rather than casting a shadow?
Runners have long debated the difference between training on a treadmill and training on solid ground.
Diamonds can also be produced synthetically in a HPHT method which approximately simulates the conditions in the Earth's mantle.