Here is a description from T. Driscol a veneer co. in the UK.
"Despite the normally featureless appearance of birch, changes in appearance occasionally take place due to beetle attack. An insect known as Agromyzia carbonaria is usually responsible, its larvae burrowing in the cambium of the tree create what are known as pith flecks, pith-like cells which cause dark markings in the wood, and localized grain disturbances. Logs that have been severely attacked, when rotary peeled, can produce highly decorative veneer known as masur birch, the irregular dark markings appearing to stand out in relief against the silky white background."