This hat is named after Xanthoparmelia, a particularly pretty form of lichen, in homage to the Lichen and Lace yarn used to knit it! I'm not sure if this is one of the many types of lichen that can be used to dye yarn (see photos in my book Spin to Knit for some of the gorgeous purples you can get from lichen dyes), but it's definitely one you'll see often in the forests near me.
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