This tough, flat, whitish, oval to kidney-shaped polypore's cap grows 1-10 inches across. Its margin is slightly inrolled, and its color darkens with age.
The recessed pore surface is also white, and the pores are too small to see without magnification. The stem, if present, is a stub.
The mushroom parasitizes, kills, and decomposes birch trees. You can find it anywhere in the US where birches grow, and it's around all year, because it's too woody to decompose quickly (it's also way too tough to eat.
People used as a strop, to sharpen razors, and they've been using it to help start fires for a very long time:
When hikers reported a murder victim they had come across in the Alps in 1991, it turned out to have been another hiker, only he had fallen into a crevasse and frozen to death 5,300 years ago!
Otzi the Iceman, as he came to be known, gave archeologists their best view of life in during the Ice Age. And among his clothing and equipment, they found some powdered birch polypore, which he had used as tinder. And after another decade, they noticed an arrowhead in his shoulder showing that he had, in fact, been murdered!