Serpentine is a mineral that was mined in northern Germany.
In the Middle Ages it was generally believed that serpentine stone had a healing effect. Drinking from serpentine objects had a detoxifying effect.
On the tin cap the maker's stamp. Initial F. I. Franz Jals, master pewter since 1699, died after 1742.
For the pewter mark see:
Literature: Süddeutsche Zinngiesser by Erwin Hintze, p.258.