September 1999 Meeting

Iridescence color of the Pinctada Margaritifara shell

   The Optical Society of San Diego is pleased to begin our 99-00 season with a talk by Mr. Yan Liu of the Gemological Institute of America (Carlsbad), with a preceding lab tour at the GIA. This talk will closely follow his paper of the same title published in the on-line journal of the OSA, "Optics Express." See the paper in the March 1, 1999 issue at http://epubs.osa.org/opticsexpress/topbiframe.htm or through www.osa.org.

Abstract: Shells and pearls often show iridescence color. The cause of this phenomenon has been attributed to diffraction, both diffraction and interference, or interference alone. We used a shell of the mollusk Pinctada Margaritifera, which shows very strong iridescence colors, to study how this color is produced in the layers of nacre in shells. From observations with a scanning electron microscope (SEM), this particular shell exhibits a very fine-scale diffraction grating structure. This suggests that the iridescence color is caused by diffraction, which was demonstrated by an experiment using an argon ion laser illuminating the shell to produce a distinct diffraction image. The strength of the iridescence color can be correlated to both the groove density of the diffraction grating formed by the shell, and the surface quality of the grooves themselves. A shell with a high groove density and a smooth groove surface produces a strong iridescence color.

Biography: Yan Liu is a research associate with the Gemological Institute of America. He has a pair of M.S. degrees, one in optics and a second in color science. His current research interests include causes of color in gemstones, color measurement of faceted gemstones, color grading of gemstones, gemological optics and color vision.

OSSD September Meeting Review
by Warren Smith

Our September meeting featured a tour of the Gemological Institute of America (GIA) research laboratory and an after dinner talk on the iridescence colors of a mollusk shell.

The GIA is a large campus with an extensive training program for jewelers and gemologists, a gemstone certification facility and a research lab where the characteristics of gemstones are studied. The lab is equipped with specialized spectrometers, fluorescence spectrographs, x-ray and radiation analysis equipment and is quite interested in finding ways to differentiate between "natural" stones and those which are created or artificially modified. Especially interesting were the tour comments by Dr. James Shigley (GIA research director) regarding a new diamond simulation by silicon carbide, who's thermal characteristics are close enough to diamond that the test currently used by most jewelers can be confused. Another point of interest was that a green diamond can be produced by radiation; since a natural" green diamond may be worth one- or two-thousand dollars per carat, vs. considerably less for an irradiated stone, one can see why there is intense interest in differentiating between the two.

The iridescence colors found in nature in shells, feathers, insects and the like have long been attributed to interference effects between thin layers. The talk by Yan Liu describing his studies of a particular mollusk, Pinctada Margaritifara, showed quite conclusively that the very strong iridescence colors displayed by this shell result from diffraction pattern produced with monochromatic light which showed the zero order and strong images of what looked (to my uncalibrated eye) to be about the second order and possibly the seventh, eighth, and ninth orders, with the other orders quite well suppressed. This would seem to indicate that not only was diffraction effective in producing the colors, but that the shell structure was functioning like an usually blazed grating. When one remembers that the chromatic dispersion in the higher orders is quite large, it is apparent that some pronounced colors can be produced.