It was narrated from Anas that:

The Prophet's ring was made of silver and its stone (Fass) was made of silver too.

أَخْبَرَنَا أَبُو بَكْرِ بْنُ عَلِيٍّ، قَالَ حَدَّثَنَا أُمَيَّةُ بْنُ بِسْطَامٍ، قَالَ حَدَّثَنَا مُعْتَمِرٌ، قَالَ سَمِعْتُ حُمَيْدًا، عَنْ أَنَسٍ، أَنَّ النَّبِيَّ ﷺ كَانَ خَاتَمُهُ مِنْ وَرِقٍ فَصُّهُ مِنْهُ .