Oakmoss

This lichen has been one of the most crucial ingredients in perfumery, acting both as a strong fixative as well as lending a unique earthy, velvety feel. Oakmoss is traditionally the foundation of the chypre and the fougère families. 

Due to it being flagged as a potential irritant, the use of oakmoss is limited by the direction of the International Fragrance Association. Some workarounds exist, however, and safe natural oakmoss can now be acquired (the irritants can be removed via distillation techniques).