BACKGROUND  Chameleon diamond is a type of color diamond with photochromic and thermochromic phenomenon. If the chameleon diamond is placed in a dark room or heated for a long time, it can change from greenish yellow to fancy yellow, and return to its original color when it cools. Previous studies on chameleon diamond focus mainly on spectral characteristics. Through testing a large number of samples, the gemological and spectral characteristics of chameleon diamond are summarized. The characteristics of chameleon diamond, include a persistent yellow phosphorescence, a 480nm absorption band, and mainly the A-aggregate of nitrogen. A model to explain the thermochromic and photochromic phenomenon of chameleon diamond was proposed.　　The characteristics of chameleon diamond discovered by previous studies are also found in a few yellow diamonds. Diamonds colored by the 480nm band show yellow fluorescence and phosphorescence under short wave ultraviolet light, low nitrogen content, high concentrations of defects related to hydrogen and nickel, but no obvious thermochromic and photochromic phenomena. Previous studies of the chameleon diamond and similar yellow diamonds were conducted independently and did not compare the two similar diamonds.

OBJECTIVES To ascertain the difference of spectral characteristics and defect types between chameleon diamonds and similar yellow diamonds, and then analyze the causes of thermochromic phenomenon of chameleon diamond to gain a deeper understanding of the possible structure of the center responsible for the chameleon effect.

METHODS Firstly, two chameleon diamond samples and four similar yellow diamond samples were collected to observe the color changes of the diamonds before and after heating. Top illumination and scattering illumination were used to observe the internal characteristics and color distribution of the samples. The fluorescence and phosphorescence characteristics of diamonds at long wave (365nm) and short wave (254nm) were observed by ordinary fluorescent lamp. The fluorescence and phosphorescent characteristics and growth structure of the samples under ultra-violet light were observed by DiamondView^TM. Secondly, in order to investigate the causes of thermochromism in chameleon diamond, the absorption characteristics of diamond samples in the range of ultraviolet visible light were collected before and after heating and compared with those of similar yellow diamonds. The differences in absorption spectra between chameleon diamond and similar yellow diamond were analyzed, and the causes of thermochromic phenomena were explored. Finally, an infrared spectrometer was used to collect the infrared spectrum of diamond samples and analysis of the types and impurity elements of diamond samples was conducted. Through the comparative analysis of the infrared spectrum of samples, the differences in the types and contents of nitrogen and hydrogen impurities between chameleon diamond and similar yellow diamond were obtained. The laser Raman spectrometer was used to collect PL spectra of the samples at liquid nitrogen temperature by using 473nm, 532nm and 830nm lasers. By comparing the PL characteristics of chameleon diamond and similar yellow diamond, the differences between the two defects were obtained.

RESULTS The color distribution of the 6 diamond samples is uniform. After heating, the chameleon diamond samples change color from yellow-green to yellow, while similar yellow diamonds have no obvious color change. Under long and short wavelength UV light, the samples are yellow fluorescence without phosphorescence. The performance of the samples in this study is consistent under long and short wavelength ultraviolet light, but it is different from the previous study results that the chameleon diamond has persistent yellow phosphorescence, which further indicates that the chameleon effect is not directly related to the phosphorescence. Under ultra-violet light, the samples have green fluorescence with irregular patterns which are markedly similar to diamonds colored by the 480nm band. Chameleon diamonds show moderate green phosphorescence.　　All the samples have C defect absorption continuum combined with 480nm absorption band. The specific wide absorption band of 650-800nm is obvious in chameleon diamond. After being heated, the 480nm and 650-800nm absorption band of chameleon diamond completely disappears. However, the 480nm absorption band of similar yellow diamonds is weakened or has no obvious change. Therefore, the poor thermal stability of 480nm and 650-800nm absorption band is responsible for the thermochromic phenomenon of chameleon diamonds. 　　The infrared absorption characteristics of chameleon diamond mainly show A-defect absorption. Isolated nitrogen and hydrogen-related features can be seen, in addition to the characteristic 1430cm⁻¹ wide absorption, which has a different origin than the platelet-related 1430cm⁻¹ feature in Cape diamond. The similar yellow diamond has higher nitrogen content, and the degree of nitrogen aggregation is higher, and does not show any type Ib character. The broadband emission of all samples occurs mainly as one band centered at 700nm, this band consists of a dozen of peaks from 595nm to 725nm in steps of about 10nm. Additionally sharp peaks at 753, 771, 799, 818, 838, 845nm are present in the chameleon diamond samples. This spectrum is virtually identical to that reported previously for diamonds containing the 480nm absorption band. Chameleon diamonds and similar yellow diamonds all contain nitrogen-vacancy, nickel-nitrogen related defects, but the amount of defects is different. Chameleon diamonds cannot be effectively distinguished based on the PL spectral. The PL spectrum analysis shows that the nickel impurity is responsible for the green fluorescence of the samples under ultra-violet light.

CONCLUSIONS This study focuses on the comparative analysis of gemology, UV-visible absorption, infrared absorption and photoluminescence characteristics of chameleon diamond and its similar yellow diamond. Chameleon diamond is type Ia diamond with low concentration of A defect, and shows some type Ib character, such as 1240cm⁻¹ absorption. Chameleon diamond has 480nm and 650-800nm absorption bands, that are responsible for the chameleon effect. The characteristics of chameleon diamond in this study are consistent with those in previous studies. Absorption bands of 650-800nm and absorption peaks of 1430cm⁻¹ (unrelated to platelet) are the main differences between the chameleon diamond and similar yellow diamond sample. It is inferred that the absorption band of 650-800nm and the absorption peak of 1430cm⁻¹ are related to the center causing the thermochromic phenomenon. Although similar yellow diamond samples also have a 480nm center, their thermal stability is different from that of chameleon diamonds. The reason for this phenomenon is unknown, therefore further studies are needed.