This form of fungi is a polypore known as Ganoderma lucidum. Presence of these conks around the base of a tree suggest internal decay, the extent of which should be investigated further to determine degree of risk for failure. So while they are beautiful, they're not such a great sign... However, they have been shown to have healing properties and are produced for herbal supplements.
The fruiting bodies themselves vary greatly in color and shape. For this piece I went with the more brightly colored variation, but you will likely see more variations throughout this project. I've been working on multiple forms of Ganoderma over the past few months, tweaking the pattern and colorwork and trying to come up with a result that I'm happy with. This one is getting there, but I still have some work to do.
By the way, if you ever see these conks in the woods you can break them off and bring them home for decor. The tissue is very woody and will not get smooshy or oozy or anything like that - I've got two on my fireplace mantel that I've had since 2007!