X-Ray Examination

Although the kind of X-ray examination of most value to gemology requires the destruction of a small amount of the stone it remains the most important single key to identification. When a beam of X-rays is directed at a solid, much of it passes through the solid with no alteration, some of it is scattered, and some is converted to heat or other kinds of energy. It is the scattered X-rays that are of interest because they are the ones that have hit atoms on the way through. A picture of these scattered X-rays is taken by proper placement of a photographic plate. A large number of atoms, all uniformly spaced and placed in the structure, will scatter the X-rays in the same direction and reinforce their image on the film. The others are scattered in various directions and produce no combined mark on the film. This means that every different plane of atoms in the structure leaves its print on the film. By careful measurement of the markings on the film and suitable mathematical treatment of the measurements, the entire internal atomic structure is revealed.
The X-ray method most often used is the powder diffraction procedure. The camera is a flat hollow metal cylinder, one end of which is a removable lid. It is very carefully machined to exact dimensions, since its uniformity and the size of its diameter are crucial in the final film measurement. There is a hole on one curved side for the entrance of X-rays, and a hole opposite for the exit of most of them. Tapered metal tubes fit in these entrance and exit ports to guide the X-rays to and from the sample. The sample is mounted on a rotating spindle at the exact center of the camera. The film is a long, narrow strip that fits flat against the inside wall of the cylinder. It has two holes that fit over the entrance and exit port tubes.
In operation, the sample—a tiny bit of the mineral which has been powdered and then held together by various adhesives—is mounted, the film is loaded in the darkroom, and the lid is replaced. As the sample spindle is turned by a motor-driven belt, the entrance port of the camera is placed against the X-ray source and the exposure proceeds, taking several hours. When the film is developed, it has a series of matched curved lines running across it. These lines represent atomic planes; for each species their pattern is characteristic and will be different from that of any other species. The films can be indexed and filed and used much the same way as fingerprints.
All told, then, in their investigation and study of gemstone species and gems through the years, mineralogists and gemologists have assembled a rather impressive arsenal of instruments and techniques. The accumulation of facts has also proceeded steadily until we have reached a point where the many problems of gemstone identification and gem preparation are matched by sufficient knowledge to solve them. It is a very rare or unusual natural gem material that worries a qualified and competent gemologist. However, man is also perfecting his ability to manufacture gemstones. It is obvious then that the science of natural gem-stones and diamond engagement rings is essential if the distinctions between natural and man-made gems are not to be obscured.