Yarrow blue essential oil is steam-distilled from the flowering aerial parts of Achillea millefolium, a rhizomatous perennial of the Asteraceae family native to temperate regions of the Northern Hemisphere whose use as a wound herb and anti-inflammatory agent is documented from the Neolithic period through Achilles' legendary use on the battlefield — the genus name commemorating this mythological association. The oil's defining characteristic is its intense blue color, produced by chamazulene generated in situ from the sesquiterpene precursor matricine during the high-temperature steam distillation process; chamazulene concentration varies between chemotypes but is responsible for both the striking color and a significant portion of the oil's anti-inflammatory activity. Chemical composition also includes camphor, borneol, sabinene, and b-pinene, and research confirms antifungal activity against Candida albicans clinical isolates, with Achillea millefolium essential oil showing meaningful inhibition zones in comparative testing against pathogenic yeasts isolated from denture stomatitis patients.^[1] A separate study characterized the oil's chemical composition and confirmed toxicity against agricultural pest insects, documenting the breadth of its bioactive sesquiterpene chemistry.^[2]