近年来因为高品质银灰色Akoya珍珠备受青睐, 大量改色处理的银灰色珍珠涌入市场, 造成混乱, 其中辐照处理改色的珍珠很难鉴别, 成为检测难题。 对一批白色和浅黄色Akoya珍珠进行了不同剂量的γ射线辐照改色实验, 并对辐照前后的样品和天然呈银灰色的Akoya珍珠进行放大观察及光致发光光谱、 紫外可见光光谱和三维荧光光谱等无损谱学测试。 实验结果表明: 浅黄色Akoya辐照的改色效果明显好于白色者, 随辐照剂量的加大, 改变的颜色加深。 通过对比辐照改色前后的样品与天然呈银灰色Akoya珍珠样品的镜下特征, 发现: 辐照改色者具有浅色珠层和带有深褐色条纹的褐色珠核, 天然呈色者则在浅色珠层和白色珠核间有一褐色有机质夹层, 有机质不均匀的地方在珍珠表面形成“黑斑”。 对比辐照前后样品与天然呈银灰色Akoya珍珠样品的谱学特征发现: 辐照改色者较改色前及天然呈色者的光致发光光谱的荧光背景更高, 但背景峰与文石主峰强度比值F/A值(1.34～1.98)比天然呈色者(0.52～1.12)略高； 辐照改色者较改色前紫外可见光谱反射率明显降低, 在紫外光区的360 nm处出现宽缓吸收, 而天然呈色者在430～530 nm范围内有宽缓吸收, 且随伴色不同位置发生改变, 有时在750～800 nm也可以有弱的宽缓吸收； 辐照改色者三维荧光光谱与改色前样品的发光中心一致, 只是荧光强度较改色前降低了一半, 但改色者与天然呈色者的主要发光中心完全不同, 辐照改色者三维荧光光谱显示两个最高强度的发光中心于Ex/Em为374/449和463 nm处, 而天然呈色者的最强发光中心在Ex/Em为280/340 nm处。 研究结果表明, 珍珠孔眼及表面特征和三维荧光光谱的测试结果, 可以很好地鉴别天然呈色和γ射线辐照改色的银灰色Akoya珍珠, 光致发光光谱和紫外可见光光谱可以作为辅助鉴定依据。

Recently, a high-quality gray Akoya pearl has been so popular in the market that many treated gray pearls appeared, resulting in a mess. Among them, the color of pearls changed by γ-ray irradiation treatment is the most difficult to identify. Therefore, distinguishing the origin of those gray pearls has become an urgent problem to solve. We chose a few white and light yellow Akoya pearls as samples to be treated by different doses of γ -ray irradiation. By observing the surface and drilled holes of these samples, the non-destructive spectroscopic test included photoluminescence spectra, UV-Vis spectra, and 3D fluorescence spectra to test and compare those unirradiated, irradiated, and naturally colored gray Akoya pearls. By comparing the amplification characteristics of samples, it is found that γ-ray irradiation samples have a light nacre and deep brown nucleus. In contrast, naturally colored ones show a brown interlayer of organic matter between a light nacre layer and white nucleus, and sometimes a “black spot” resulted from the concentrated area of organic matter. By comparing the spectroscopy characteristics of the samples, it is found that the fluorescence background of the photoluminescence spectra of the irradiated samples is higher than that of unirradiated and naturally colored ones. Their F/A value (1.34~1.98), the intensity ratio between the background peak and aragonite main peak, is slightly higher than that of naturally colored ones (0.52~1.12). The reflectance of UV-Vis spectra of irradiated samples is lower than unirradiated ones, absorbed at 360 nm in the UV region. In comparison, the absorption valley of the naturally colored samples located in the range of visible light at 430~530 nm, which changes with different overtone, sometimes there is a weak wide and weak absorption at 750~800 nm. 3D fluorescence spectra show that the luminescence center of irradiated and unirradiated samples is consistent, except fluorescence intensity reduces by half compared with unirradiated ones. However, the main luminescence center of the irradiated pearls is completely different from that of the naturally colored gray pearls. The strongest fluorescence center in 3D fluorescence spectra of γ-ray irradiated Akoya pearls is at Ex/Em of 374/449 nm and 463 min the form of double peaks, while that of the naturally colored gray Akoya pearl is at Ex/Em of 286 nm/340 nm. In a word, combined with the observation from the surface and the drill hole and 3D fluorescence spectra, it is well to distinguish between naturally colored gray Akoya pearls and γ -ray irradiated gray Akoya pearls. In contrast, the F/A value of photoluminescence spectra and UV-Vis spectra can be used as an auxiliary identification.