When it was first announced, I read that it was a series of phosphors embedded in the metal. As the coin wore with age, different ones would become detectable a

When it was first announced, I read that it was a series of phosphors embedded in the metal. As the coin wore with age, different ones would become detectable at the surface. Obviously I have no idea of the veracity of that, or the accuracy of my memory of the potentially fictitious article.