Your quote: "Certain cloudy...rusty-looking". That's right! Recently chipped opaque amber beads of a nice rusty colour can be seen to be made of a bright yellow stuff, under the aprox. 1 mm. thick brownish crust...

I do not fully agree with your "In fact...to aging". I do not know what causes transparent amber to darken and turn more opaque, but in my experience dark, opaque beads -- the largest bead in Carl's strand would be a typical one -- even beads looking hoplessly damaged due to their rough surface, become less opaque after a rubbing with raw shea butter. Amber initially opaque will turn of a richer shade, but will not gain translucence, no matter how much we massage it! I guess there are two types of opacity: one that is congenital to the amber itself and due the impurities, etc. of the material, another one caused by aging (i.e. weathering** of the surface).

One more point to consider: some beads are partly opaque, partly translucent, and that can best be seen after the "treatement".

**specially "sanding", because I have only found this caracteristic weathered surface in beads from Northwest Africa (Sahara area), never for example in Tibetan beads.