Dagarful, also known as Dagar Phool, Black Stone Flower or Kalpasi, is a lichen used in Indian cuisine. It has a flavour similar to cinnamon and you can use that as a substitute if you have to.
I have to agree with Rick Stein though, it is transformational and brings something to curries that I can’t quite put my finger on. In Australia it really isn’t hard to source and it is so worth the effort.
Give it a try,