My Google-Fu is good today..I found this info on the web:
Siberians and other northern peoples in Asia, Europe, and North America for centuries have highly prized chaga for its great medicinal and curative powers. Those of us who know its value seek it out when traveling in the northern forest.
Chaga is a parasitic carpophore that enters a wound on a mature tree, usually birch. The chaga grows under the bark and erupts into a grotesque black charcoal-like conk on the tree trunk; hence the Latin term "Obliquus" in its scientific name.
The Chaga conk grows with the birch tree for five to seven years during which time it absorbs nutrients and phytochemicals from the wood. When the chaga conk flower ripens it falls to the forest floor. Usually the host tree then dies, completing a 20 year cycle.
It is estimated that only about 0.025% of trees, only a few of every ten thousand, will grow a chaga conk. This makes the chaga mushroom somewhat rare even in its prime northern range.
Identifying Chaga Mushrooms
Chaga Conk
The Chaga mushroom is a fungi that grows on the wounds of birches. Occasionally chaga is also found on ironwood, elm, alder and beeches but both paper birch and yellow birch seems to be its favorite.
This Chaga tinder fungus I found while snowshoeing yesterday is growing on a large yellow-birch tree that had been damaged years ago during logging operations. The outside of this easy to identify mushroom somewhat resembles the charred remains of burnt wood, being black and crumbly.
Of irregular shape, the inside of a chaga is the color of rusted iron or yellowish with white or cream colored veins. It is corky of texture and tends to become lighter in color closest to the tree. You can see the colors in third picture where I have pulled the chaga off the yellow birch tree using my tomahawk.
Chaga is known as a polypore fungus, which means it has pores instead of gills. The chaga mushroom does not hold a great deal of water as does other types of mushrooms. As the chaga conk grows its outside dries out, turns black, and cracks. I have seen large Chagas well over three feet (one meter) in length and one foot (.33 meter) thick.
The chaga mushroom is commonly known as the true “tinder fungus” for its use in building fires. In fact, chaga is the true tinder fungus, as opposed to the false tinder fungus which is shelf-like in shape and does not crumble.