This is a photo of a toppled over stump in my backyard. The darker rings around the inside of the stump are known as heartwood. Heartwood is the older, dead, central wood of a tree, which is generally darker and firmer than the younger wood. Heartwood can also be referred to as the duramen.
Heartwood is as a result of a chemical transformation that happens naturally, and has become more resistant to decay. Heartwood forms randomly and it is not a programmed process. Once heartwood formation is complete, the heartwood is no longer living.