Is it wear on the bottom? - or just a very rough finish?
They're doing a lot of stuff with strappings just now - many with a sort of open weave effect like the base of this.
The iridesence is an all over shine on this piece, applied afterwards, while iridesence on Mdina is most often the result of an escaping silver ion gas from the piece (silver chloride is used as a colouring agent) and depositing itself on the outside where you end up with a yellowy, bluey silvery effect- it's not (normally) an all-over thing.
This is an image of a Fish, which shows Mdina iridesence quite nicely.