About Parmelia saxatilis (L.) Ach.
This lichen, Parmelia saxatilis (L.) Ach., has a greenish-gray to bluish-gray thallus that may turn brown when growing in exposed locations. It most commonly grows on rock, and can occasionally be found growing on bark or wood instead. Parmelia saxatilis is used to produce dyes that yield deep red-brown and rusty-orange colors. In Scotland, this species is called "crotal", and it was historically used to dye traditional cloths, including Harris tweed. The Parmelia sulcata group, a somewhat similar species complex with a cosmopolitan distribution, generally grows on trees.
