Objective To analyze the reasons for the appearance of special-shaped oil drop image in spot method, broaden the applicability of the spot method, and improve detection efficiency. Methods Ten typical samples with different combinations of facet, cabochon, polished, and frosted were tested by a gem refractometer. The study focused on abnormal oil drop images whichhas no clear boundary between light and dark areas but variations in light and dark areas, as well as the cases that the abnormal oil droplet image did not show changes in light and dark areas but only a small bright spot moving along with the oil droplet image. Results It was indicated that even when abnormal oil droplet images are observed in Spot method, reliable refractive index values of gemstones can still be rapidly obtained. Conclusion This study broadens the applicability of Spot method to obtain refractive index for faceted, fine frosted gemstone, and the determination of refractive indices beyond the range of the refractometer in three specific scenarios.

refractometer  /  abnormal spot method  /  reading