This is referred in the description 'the tool to attack diseases is called Yakuseki, and stones were used as acus in old ages while iron were used in modern ages for healing patients.' in the eighteenth version of 'Genno ongi' (annotation of Buddhist terms).
「玄応音義」巻18に「病を攻むるを薬石といふ、古人石を以て針となし、今人鉄を以てす、皆な病者を療す」とあるのはこれである。