Here's some blurb on masur birch, lifted from my website!
Masur birch is not a specific wood species, it is a birch (Betula pendula) found mainly in Scandinavia, Russia and Belarus that has a distinct figure caused by the attack of the Agromyzia carbonaria beetle. The beetle larvae burrow into the cambium (the tissue between the sapwood and the bark) of the tree and the tree repairs the damage as it grows, thereby creating pith flecks and dark markings which results in a highly decorative figure. Masur birch is often referred to as Karelian birch burl, named for the Karelia area in Finland where trees with this type of figure are more prevalent.
It's a wood I like to turn too, one with plenty of figure and character!