常见灰色珍珠的紫外可见漫反射光谱类型及其灰色成因

基于紫外可见（UV-Vis）漫反射光谱就当前市售流通领域常见的灰色珍珠品类进行表征并依据样品的反射谱图的类同特征予以分类，并初步探究了灰色珍珠的颜色成因。研究表明：(1)基于灰色珍珠的UV-Vis 漫反射光谱中约280 nm 处吸收峰的有无，首次将灰色珍珠分为Ⅰ型（存在明显的吸收）与Ⅱ型（无吸收或仅存在较弱的吸收）。并据Ⅰ型珍珠在其紫外可见光区的反射峰形与其反射主波长位置的差异进一步分为Ⅰn, Ⅰp与Ⅰf三个亚型，上述Ⅰ型珍珠均为有核珍珠。其中Ⅰp型珍珠的内核为白色、内核与珍珠层之间较多存在褐色、黑褐色的过渡层，该过渡层可能是导致珍珠呈灰色的直接原因。与此同时，上述Ⅰp型珍珠在宝玉石鉴定领域一般认为是未经处理的。（2）结合前人就珍珠辐照的相关工作及本工作中贝壳珍珠层辐照前后颜色的改变及UV-Vis反射光谱的变化特征, 推断辐照仍是人工处理导致珍珠呈现灰色主要原因之一。基于样品对应的UV-Vis反射光谱中约280 nm吸收峰的消失或仅呈现一吸收肩可初步定性该类珍珠经优化处理。同时，从Ⅱ类灰色珍珠的断面结构看，当前灰色珍珠并不仅局限于有核珍珠，无核灰色珍珠同样存在于流通领域。课题研究工作可为灰色珍珠及其优化处理品的鉴定提供理论与技术支撑。

The Types of UV-Vis Diffuse Reflectance Spectra of Common Gray Pearls and Their Coloring Mechanism

The spectral types of usual gray pearls were classified based on UV-Vis diffuse reflection spectra. The coloring mechanism of gray pearl was further investigated. The results show that :(1) based on the presence or absence of absorption peak nearby 280 nm in the reflectance spectra of gray pearls, which are firstly divided into typeⅠ (obvious absorption) andⅡ (no absorption or weak absorption). Furthermore, according to the shape of spectra and the position of the main wavelength of the reflection peak, the type Ⅰ pearls are further divided into three subtypes: Ⅰn, Ⅰp andⅠf. Among them, the color of the nucleus of typeⅠp pearl is white, and a brown or blackish-brown transition layer exists between the nucleus and nacre, which may be the direct cause of the gray color of the pearl. Meanwhile, the type ofⅠp pearls are generally considered to be untreated in the field of gemstone identification. (2) because of the change in the color of nacre before and after irradiation and the variation characteristics of the UV-Vis reflection spectrum in this work and the previous work on irradiated pearls, it is preliminarily concluded that irradiation is still one of the main reasons for the coloring mechanism of gray pearls. Moreover, the pearl can be identified as having been treated based on the disappearance of the absorption peak or the presence of only one absorption shoulder near 280 nm in the corresponding reflectance spectrum. According to the typeⅡ gray pearls section structure, the nucleated and non-nucleated pearls coexist in the circulation field. A more accurate method used to identify the irradiated pearl should be developed.