Yup, the beads I heat-treated myself have the same very thin dark "skin". I can't tell if a bead from the African trade has been heat-treated or has darkened "naturally" with time, but I think the result would be the same - just a surface effect. A number of references mention that the reaction happens only if oxygen is present. One of the "fixes" to this problem was to store the phenolic item under nitrogen or vacuum to exclude oxygen- not very practical for the casual buyer of beads, cigarette holders, etc.!

The dark veins can be due to oxidation on the exposed crack surfaces, or maybe intrusion of dirt, body oils, etc. Beads that are still amber yellow have dark cracks. The cracks form as the material slowly shrinks with time - the shrinkage happens since the chemicals are still reacting although very, very slowly. Small molecules that were in the chemical mix may also continuously diffuse out at the surface and add to the shrinkage. Some beads have no sparse cracks or veins at all so I assume they were from manufacturers that tweaked the formulas to reduce or eliminate the shrinkage problem.

Another aging effect that I found when exposing some yellow phenolic beads to the intense sun here in CA - the yellow color faded to a much lighter yellow after about a year. Since there were many different commercial phenolic products I expect we will find all sorts of color stability/ instability for beads made with them.