Wednesday, October 1, 2008

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.

Gemstone Inclusions

The student gemologist, when he first begins to look inside gem-stones under magnification is often amazed at the myriad of included objects he sees there. Tadpole, comma, round, or elliptical bubbles abound as well as fibrous horsetails, cracks, beautiful tiny crystals, blades, feathers and mossy traces of matter. At magnifications of from 10 to 40 times under the microscope it is often possible to recognize inclusions that tell not only what the gem is but where in the world it came from. Tiny actinolite blades in an emerald signal immediately that it was mined in Russia and rtot at the famous mines of Colombia.
These inclusions may have arrived in the gemstone at different times. Some existed before the gemstone was formed and they were swept up and trapped in the developing solid. Even tiny droplets of the liquid from which the crystal formed are sometimes trapped. Some liquid-filled inclusion cavities have tiny gas bubbles that move back and forth in their small prisons as the stone is tilted and other diamond jewelry. Now and then, a mineral species developing from the same liquid as the gemstone leaves its trace as a scatter of bright, little but well-formed crystals peppered through the stone. Often, too, after the gemstone has formed it develops a series of tiny cracks and fissures. These may later be filled by the infiltration of liquids which form new crystalline material to "heal" the breakage. This accounts for the typical "healed" feathers seen in Ceylon sapphires. There are certain significant internal features caused by accidents during growth. Color zoning, sometimes not too obvious without magnification, will appear as definite bands of differing color intensity due to interruptions during growth or slight changes in the content of the supply of material brought to the forming crystal. Prominent hexagonal color zoning is typical of Burmese sapphire.

Spectrum Analysis

For colored gemstones it is often possible to obtain very useful information for identification by use of the gem spectroscope. The instrument's operation is based on the separation of white light into its complete rainbow, or spectrum, of colors. This is done by a built-in prism which receives the light through a narrow slit. The prism sorts out the various wavelengths by its strong dispersion. Often a diffraction grating is used instead of a prism to diffract or separate the colors. Looking through the opposite end of the instrument one can see the continuous rainbow as a band of touching parallel bars or lines of different colors. They range from violet and indigo at one end, through blue, green, yellow, orange, to red at the other end. Now the colored gem is placed between the light source and the spectroscope slit. As expected, certain specific colors from the white light are absorbed by the gem and do not enter the spectroscope. The result can be seen in various white gold engagement rings. Their absence causes black bars—the absence of specific colors—to appear in the continuous spectrum of color bars. For some colored gems these patterns of black bars are very distinctive and are good identification features.