Metallic poison was something that could be refined from any metal. However, the more precious the metal was, the more potent and high-leveled the poison would be.
The gold coins circulating in the world consisted of gold that had been refined. There was next to nothing else to be extracted from them, just the purest of gold. However, that was not the case with ancient gold coins.
As such, when mages wanted to refine metallic poison, they would purchase ancient gold coins. That was the reason why one ancient gold coin was worth a hundred of their contemporary counterparts.
Metallic poisons were not meant for killing people; they were an essential ingredient for corrosive etching of magic armors. A set of magic armor without the use of metallic poisons in their composition would need the use of mithril or metallic poisons to etch magic arrays. There were no other ways of doing it.
