Aoshima (1951) Inonotus obliquus, commonly called chaga (/ˈtʃɑːɡə/; a Latinisation of the Russian word ча́га), is a fungus in the family Hymenochaetaceae.

It is not the fruiting body of the fungus, but a sclerotium or mass of mycelium, mostly black because of a substantial amount of melanin.

The chaga spores enter the tree through wounds, particularly poorly healed branch stubs.

During the infection cycle, penetration of the sapwood occurs only around the sterile exterior mycelium mass.

While the tree is alive, only sterile mycelial masses are produced (the black exterior conk).

[7][8] Chaga contains extremely high concentrations of oxalate, 2800–11200 mg total oxalates/100 g sclerotium, one of the highest reported in any organism.

[6] Inonotus obliquus is found most commonly in the Circumboreal Region of the Northern Hemisphere, where it is distributed in birch forests.

[10] Attempts at cultivating this fungus on potato dextrose agar and other simulated media resulted in a reduced and markedly different production of metabolites.

[11][12] Cultivated chaga developed a reduced number of phytosterols, particularly lanosterol, an intermediate in the synthesis of ergosterol and lanostane-type triterpenes.

However, caution is warranted with chronic use due to the extremely high concentrations of oxalates in chaga.

According to Potawatomi biologist Robin Wall Kimmerer, "Once an ember meets shkitagen it will not go out but smolders slowly in the fungal matrix, holding its heat.

Even the smallest spark, so fleeting and easily lost, will be held and nurtured if it lands on a cube of shkitagen.

"[16] The name chaga comes from the Russian name of the fungus, ча́га, čága, which in turn is borrowed from the word for "mushroom" in Komi, тшак, tšak, the language of the indigenous peoples in the Kama River Basin, west of the Ural Mountains.