The Alchemists knew their secrets were highly desired by those that sought their downfall. So they carried their most powerful weapons around their necks, to be thrown into the fire should the worst ever come to pass so at least that way they'd take the entire building with them. The glow that the necklace cast worked to inform would be cutthroats what may await them should they make such a foolhardy attempt.
Bottle shape. See Pixie's Glowing Necklace for teardrop shape version.