Diffraction Of Gems

Although it resembles interference color, the peacock play of colors in opal, which is also pseudochromatic, arises from still another process—one which has not been understood until recently, when it became possible to take electron microscope photographs of up to 40,000 magnifications. In these photographs, precious opal is seen to consist of layer upon layer of silica spheres (Si02) arranged row upon row in neat, orderly grid patterns with relatively uniform spacings between the spheres.

This arrangement acts like the optical-laboratory device called a diffraction grating. This is usually made by scratching a series of fine parallel lines on a glass or metal plate with a diamond point. The lines are spaced as many as 30,000 to an inch. Portions of a light beam directed at such a grating are reflected back from each of the thousands of polished gaps between the scratched lines. Using just one of these tiny "beamlets" as an example, the part that is closest to the edge of a neighboring scratch is bent from its expected path. This bent or diffracted portion is now thrown out of phase with the rest of the beam-let and is in a position to cause interference with its neighboring light waves. Also check princess diamond earrings

The behavior of this single tiny reflection is repeated by all the thousands of others, giving a uniform interference color all across the grating. Precious opal shows its diffraction colors in patches. This is because the grating-like arrangement of silica spheres occurs in irregular patches and the patches are not necessarily oriented in the same direction. The thickness and spacing of the scratched lines of a diffraction grating have a direct effect on the interference colors produced. The relative positions of the light source, the grating, and the observer also help to determine the colors. So it is with viewing opal. The size and spacing of the silica spheres and the relative positions of the light source, the opal, and the observer make striking differences in the pseudochromatic colors seen.