A diamond wafer is useful for producing an x-ray beam.
When a beam of high-energy electrons passes through a diamond wafer, it loses energy in the form of x-rays, or photons.  These x-rays can be used to look deep inside the core of an atom, just as you can probe a bag of marbles by poking your finger into it.
If we use a nearly perfect diamond, we’ll get a high-quality x-ray beam.  Owing to the regularity of the crystal structure, our x-ray beam will be in phase and have a single wavelength, just like a laser.  This allows us to see as clearly as possible into the atomic nucleus.