Iyas bin al-Harith bin al-Muaiqib quoting his grandfather said and his grandfather from his mother's side was Abu Dhubab: The signet-ring of the Prophet ﷺ was of iron polished with silver. Sometimes it remained in my possession. Al-Mu'ayqib was in charge of the signet-ring of the Prophet ﷺ.
Benefits and Issues: The material with which an iron ring is plated will take the same ruling as that material, whether it is gold or silver. And for men, silver is permissible. And Allah knows best.
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 4224
´Iyas bin Al-Harith bin Al-Mu'aiqib narrated that his grandfather Mu'aiqib said:` "The ring of the Prophet [SAW] was made of iron with silver twisted around it." He said: "And sometimes it was on my hand." And Mu'aiqib was the keeper of the ring of the Messenger of Allah [SAW].
Hafiz Muhammad Ameen
(1) The purpose of the chapter heading established by Imam al-Nasa’i rahimahullah is to clarify this important issue: that it is legally permissible (shar‘an) to wear an iron ring that has a silver covering. It is necessary to avoid wearing a ring made purely of iron, because it has been described as the adornment of the people of Hell. (: al-Mawsu‘ah al-Hadithiyyah, Musnad Ahmad: 11/69).
(2) This blessed hadith also proves that serving the people of knowledge and virtue is recommended (mustahabb). Furthermore, it is permissible to have a free man perform service as well, provided that he is willing and consents to do so gladly.
(3) It is necessary to safeguard official seals, as well as those of other important institutions, which are used to stamp letters and documents, so that they are not misused by anyone. Otherwise, all such documents and letters would be rendered invalid and unreliable. And Allah knows best.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 5208