The Messenger of Allah put on a ring of silver with an Ethiopian stone (Fass).
Sunan an-Nasa'i Book 48, Hadith 240
← Previous Hadith "The Messenger of Allah wanted to write to the Romans, and they (the Companions) said: 'They do not read any letter unless it has a seal.' So he took a ring of silver and it is as if I can see its whiteness on his hand, and engraved on it (were the words): 'Muhammad Rasul Allah.'"

Sunan an-Nasa'i Book 48, Hadith 239 Next Hadith → "The ring of the Prophet was of silver, and its stone (Fass) was made of silver too."

Sunan an-Nasa'i Book 48, Hadith 241