Presumably J.K. adapted it as a noun.
More likely she knew it had a long history of use as a noun, as in Halloween = Eve of All Hallows = the day before All Saints’ Day.
In addition to the “saint” meaning, the OED records this obsolete sense: “In pl. applied to the shrines or relics of saints; the gods of the heathen or their shrines.” Her adaptation would seem to be simply from “holy relics” to “powerfully magical relics”, although some characters in the wizarding culture she created venerated them in a way analogous to religious relics.
