This particular description is somewhat puzzling. It would appear that John has coined a new term here in order to make a theological point. Of course, Christ is not an angel, but the description of Christ has been influenced by the physical description of angels in the OT, because the angels were made in the image of Christ. The addition of frankincense is significant. Only metals can be refined in a furnace.
Frankincense is the sap of an aromatic tree that grows in Arabia.
You can't refine frankincense in the fire. You can only refine metal. So the main term is clearly brass , and frankincense is merely describing the brass further. It was primarily used as incense that was offered with the grain offerings in the Levitical sacrificial system. Frankincense was to be burned with the grain offering as a sweet smelling sacrifice to the Lord.