Abstract: "Chameleon" amber is a special variety of amber from Myanmar, which has an unique optical effect. When the "chameleon" amber is observed under the white light in black background, its surface will show floating green colour. There is still no systematical research about the cause of this phenomenon. In this paper, the spectral characteristics and colourimetry were used to investigate "chameleon" amber and the cause of the special colour phenomenon. The following main conclusions are obtained: (1) "Chameleon" amber from Myanmar has Usambara effect. With the increase of thickness, its hue angle decreases, the body colour changes from yellow to red, and the saturation increases at first and then decreases. It shows that the body colour will become brighter with the increase of thickness in a certain range. If the thickness reaches a certain value (about 11 mm), the saturation will decrease rapidly and make the colour darker. (2) The floating green colour on "chameleon" amber's surface is closely related to the characteristics of the fluorescence center at 465 nm. It is the high intensity and great independence of the 465 nm fluorescence center that make the obvious floating green. In most cases, the optical phenomenon observed by human eyes is the combined effect of fluorescent colour and body colour. The Usambara effect of "chameleon" amber will have an impact on the presentation of the amber colour. (3) The relative spectral power distribution of the light source will affect the fluorescence colour of "chameleon" amber. Under D65 light source, the intensity of bluish green fluorescence excited by 430 nm is higher than blue fluorescence excited by 382 nm, so the fluorescence colour is mainly bluish green. And the radiation intensity of D65 light source in the long wavelength region is higher than that of short wavelength, which is beneficial for fluorescence colour to lean towards green tone. (4) The sunlight commonly used in the market is the laboratory simulated D65 light source, under which the phenomenon of floating green colour can be observed. When studying the cause of the colour of gems, we should consider the influence of the light source, the environment and the nature of the sample on the colour presentation effect at the same time, and draw a conclusion by comprehensive analysis.