The touchstone

The touchstone, also known as ‘lapis lydius’, is made of black flint (‘lidite’), a kind of highly compact microcrystalline flint common in the Siena area. Since ancient times it has been used to empirically determine the quantity of gold contained in an alloy and, consequently, to tell real gold from fake gold. For this reason the founders of the Accademia dei Fisiocritici chose touchstone as the emblem of their institution