Humans have used chaga for thousands of years. But its popularity is shining new light on forest medicinals. Black Gold. The Diamond of the Forest. The Mushroom of Immortality. Hearing the chaga fungus referred to by its many nicknames, it's easy to imagine sparkling gemstones and tendrils of precious metals hanging from trees. Instead, chaga protrudes parasitically on the trunk of the boreal birch; bulbous, black and more reminiscent of bark than fungi. But a fungus it is. And a powerful – or at the very least, powerfully popular – one at that. While people in Russia and parts of...