Hi Christine,
Indeed, you're most certainly right regarding high manganese content, I was somewhat astonished (and thrilled) when I pointed my UV pen at it, to see it glowing bright green when it gives off not even the faintest green glow in sunlight. It glows markedly brighter around the thicker areas, i.e. the rim and base however it does glow in its entirety in an unlit room at night.
Any suggestions regarding a designer/factory?
Cheers