Thanks a lot for your comments (and research), m

It is a mystery piece...
To answer some of your questions:
construction from outside to inside: (I think) clear casing - teal - golden iridescent crackle - dark blue - ochre (fading to green towards rim)
Dark blue only shows in the golden crackle areas.
And I really don't know why this blue layer exists; maybe you're right and it is partly responsible for the teal colour.
I attach a close-up pic of the wear to the base, which to my eye looks as I would expect in a piece of some age.
There is also a bigger blue area visible.
I would rather think it is not a studio piece.
But if it is old or new -- I cannot tell.
Michael