Scientists estimate there are a quadrillion tons of diamond below Earth's surface

The Earth contains a vast amount of diamonds, estimated to be a quadrillion tons, located over 100 miles beneath the surface under continental tectonic plates. Studies suggest that 1 to 2% of cratonic roots may contain diamonds. Researchers used virtual rock models to determine that diamonds allow sound waves to travel quickly through these roots.

What method did researchers use to test the material that allows sound waves to travel quickly?

What was the result of the research conducted through the cratonic routes?