Evaluating and identifying pearls and their nuclei by using optical coherence tomography.

Optical coherence tomography (OCT) has been utilized to evaluate pearls including their nuclei noninvasively. By visualizing the internal structure of a pearl, we could measure the thickness of its nacre layer, observe the fine sub-structure of the nacre, and inspect the nucleus through the nacre. The system also allowed us to classify pearls into beaded- and non-beaded ones; usually, the saltwater ones have nuclei even though there are beaded freshwater pears and non-beaded saltwater pearls. Any cracks, crevices, or blemishes not only in the nacre but in the nucleus of a pearl could be clearly visualized. The OCT system was based on a 20 kHz swept-source of a 1.31microm central wavelength and an 110 nm full-width-at-half-maximum (FWHM) bandwidth. To get the 2-D images all around the circumference of a pearl, the pearl was rotated by a motorized rotating stage. And to achieve 3-D volume images, galvano-scans were made along two axes. Of all things, the OCT allowed us to check the use of a forbidden nucleus, usually made of a Giant Clam shell thus fragile, without hurting the pearl. With this modality, we believe, it would be possible evaluating pearls both in qualitative and quantitative. Comparison with the images taken with an optical microscope and X-ray radiograph gives the refractive index of pearl as about 1.53 in average.