I think this is some type of lichen. I think it will grow larger and be more obviously a lichen, but it may grow very slowly.

You can see how beautiful designs like this in nature would inspire art and fabric design and lots of other things.

I agree, I think it is a very slow growing lichen.

ladyslppr and esh ga, thanks for the follow ups ,yes it may bee a slow grower,,I wonder if there is a web site to show similar lichen,if you ever find one please let me know.I was wondering if the dark green is the newer part or the lighter green,anyways thanks..Tom in Pa.

It is a lichen. The dark green is probably a moss. The lighter edges are where other things can't grow due to the enzymes in the lichen.