First off what is Chaga? Chaga is a fungus that grows mostly on birch tree's in northern climates. Its spread to new tree's through wounds in a tree, these can be broken branches, wood pecker holes, splits or cracks, etc. Once a tree is 'infected' by chaga it can continue to grow for many years without issue. Chaga will grow and 'fruit' into a black growth on the tree. Looks something like cracked lava rock.
When you harvest Chaga it does not hurt the tree, and the Chaga will continue to grow as long as the tree lives regardless of how much you harvest. It is however, best practice to leave at least 50% of the chaga on the tree for others to harvest in the future, since it is a very slow growing fungus. Many people believe that if you harvest too much it will not continue to grow which is not the case as long as at least a small bit is left on the tree and unless you are damaging the tree during harvest this will always be the case.
Here you can see what it looks like under the black 'shell' after harvesting some.
The inner part of the fungus has a cork like texture while the outside is relatively hard and crusty.
You should dry any chaga you do not use right away to prevent it from going bad or growing mold since it does have a high moisture content.
Chaga has several great uses including a healthy substitute to coffee, and many medicinal qualities. It can also be mixed with coffee or cacao.
My personal favorite is to simmer the chaga in water for a few hours, then mix with raw cacao powder, a little maple syrup and some cream for a wonderful drink. Perhaps not as healthy as just chaga tea alone which is also good and has a flavor almost like lightly sweetened vanilla.
Used or spent Chaga once dried can be used as a fire starter in your outdoors kit so don't waste good chaga without making some tea with it first.