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

Area of Science:

• Gemology
• Optical Engineering
• Materials Science

Background:

• Traditional pearl evaluation methods can be invasive or limited in scope.
• Understanding the internal structure of pearls is crucial for quality assessment and authenticity verification.

Purpose of the Study:

• To demonstrate the application of Optical Coherence Tomography (OCT) for noninvasive pearl evaluation.
• To assess the capability of OCT in visualizing pearl internal structures, including nacre and nucleus.
• To explore OCT's potential for pearl classification and defect detection.

Main Methods:

• Utilized a swept-source OCT system (1.31 µm central wavelength, 110 nm FWHM bandwidth, 20 kHz repetition rate).
• Employed a motorized rotating stage for 2D circumferential imaging and galvo-scans for 3D volume reconstruction.
• Compared OCT findings with optical microscopy and X-ray radiography.

Main Results:

• OCT successfully visualized pearl internal structures, enabling measurement of nacre thickness and sub-structure observation.
• The system differentiated between beaded and non-beaded pearls and detected internal cracks, crevices, and blemishes in both nacre and nucleus.
• Forbidden nuclei, such as those from Giant Clam shells, were identified without damaging the pearl.
• The average refractive index of pearl was determined to be approximately 1.53.

Conclusions:

• OCT provides a noninvasive, high-resolution method for comprehensive pearl evaluation.
• This technique allows for both qualitative and quantitative assessment of pearls, including authenticity checks and defect identification.
• OCT holds significant potential for advancing the gemological assessment of pearls.