Why real diamonds don't show up on x-ray?

The Development Center for X-ray Technology EZRT in Furth has developed a demonstrator that detects diamonds hiding inside rocks of volcanic origin. The department focuses on the subjects of X-ray sensor technology, computed tomography, image processing and optical 3D inspection systems as well as applications.

The process is based on dual-energy X-rays. In the process, two images of the same object are produced using two different X-ray spectra. Different substances can be identified with very high reliability. The new technology is capable of detecting diamonds of just a few millimeters in size in kimberlite ore -- of grain sizes up to 50 millimeters

Currently, the diamond industry is already using X-rays in order to find the coveted gems. The conventional process, however, can detect the diamonds only at the surface of the ore. The diamonds irradiated and activated by X-rays emit light in the optical spectrum. "With particularly pure specimens, this technology does not work because it is precisely these that do not radiate under X-ray light,". In order to find the gems nonetheless, it has so far been necessary to break up the igneous rock into very small pieces. That consumes large quantities of water and energy.

With the EZRT demo model, the crushed rocks pass through an X-ray machine at a speed of three meters per second. Both X-ray images produced provide data about the chemical atomic number of the materials -- the number of protons in the respective nucleus. Diamond is pure carbon, a relatively light-weight element with the atomic number 6. Kimberlite typically holds a conglomerate of silicates and aluminates. Depending on the excavation area and mine, the atomic numbers vary between 12 and 14. The new algorithm uses this data. It links them with the data from both X-ray images, separates the diamonds from the kimberlite, and displays the results on two separate images.

This method is not limited to the detection of diamonds. Anywhere that materials have to be identified and cleanly separated, their use is possible. Another example is the processing of industrial coal. Rocks have to be sorted out, or the ash content kept to a minimum. The X-ray's eagle eye could even find the highly coveted rare earths that are concealed in old cellphones, computers, and television sets to utilize them.