The short answer is that we are making guesses about what's going on very deep in the Earth. No man-made drill has ever really done more than scratch the surface of this planet, so most of what we do here is intended to help verify theories on the Earth's composition. This data is necessary because these materials have very different properties if you change their conditions. For example, in normal human living conditions, water's natural state is a liquid. However, it is easily observeable as a solid or a gas at different temperatures.
Liquid water is actually quite rare in the universe.
If you subject water to the temperatures and pressures found in the center of the Earth, it becomes an acid. We cannot make any assumptions about the properties of materials until we test them under appropriate conditions. We put samples of common minerals and elements under extreme pressures and temperatures, then conduct experiments to determine their properties under those conditions. By learning about the molecular properties, we can predict their behavior. Because the diamonds are clear, we can then shine lasers through the DAC to get information about the sample.