[Machine] The ring of the Messenger of Allah ﷺ was made of iron covered with silver, and the word "al-Mu'aqqib" refers to the seal of the Messenger of Allah ﷺ.  

«كَانَ خَاتَمُ رَسُولِ اللهِ ﷺ مِنْ حَدِيدٍ مَلْوِيٍّ بِفِضَّةٍ وَكَانَ الْمُعَيْقِيبُ عَلَى خَاتَمِ رَسُولِ اللهِ ﷺ» اللَّفْظُ لِأَبِي دَاوُدَ