The greenish blue color of blue amber from the Dominican Republic and Indonesia is caused by UV-stimulated fluorescence. The greenish blue fluorescence color is confined to the surface. There are two reasons for this: (1) UV light can only stimulate the greenish blue fluorescence on the
surface of amber, and (2) amber strongly absorbs the greenish blue fluorescence light and does not allow it to penetrate much below the surface. Where a sample is thicker, the cutoff wavelength of the long-wavelength pass spectrum moves to a longer wavelength, and the greenish blue fluorescence can only penetrate a very thin layer of the amber. Thus, the blue fluorescence only appears on the surface.