Genuine amber has many different colors and levels of transparency. Clear or cloudy, reddish-brown, yellow, whitish, and even blue. All can be genuine amber. Some amber materials are actually a composite of amber pieces and/or powders which have been heated and pressed together or extruded, without need for any binders or fillers. Finally, there is bonded amber that, like the pressed amber, consists of pieces or powders, but which also includes a binding agent. Both block and pressed amber will pass the various simple tests for amber because, well, they’re amber. Bonded I’m not sure about.
This stem passes the two simplest tests — static and salt water flotation. In addition, Dunhill was founded as a luxury brand. It would be out of character for them to have used anything less than natural amber. The perfect registration of the mouthpiece to the shank and the lack of evidence of the shank being refinished suggests to me that the amber is original. The only thing that gives me pause is that the dot is not at 12 o’clock. I wonder if the maker thought that it might be worse to weaken the tenon at 12:00 rather than 2:00. It’s a semi-plausible explanation if you consider that some smokers like to tap their bowls on a knocker.
Although I can’t put my hands on one at the moment, I’m certain I’ve seen amber-stemmed Dunhill pipes from the 1920s which did have a white spot on the amber, but no doubt that they put it on the tenon to avoid damaging the amber.