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Tell them they work in the food industry and they have a choice. Everything is removable.
I'm not joking.
Some sites allow jewellery to be covered, especially for example, in something like beverage where most of the product area is in pipework. It depends upon what certification schemes they work to if that's allowed. There is always one or two places in plant though where there are exposed areas and unless you have different standards for there it just becomes really awkward to control and to be honest one rule (whatever standards say) is just easier. Just like "everyone wears hair nets" is a rule that you enforce for bald people.
So I would just make it a flat rule. Explain while it's awkward in that instance and unlikely to fall out that it creates a grey area for other jewellery and for that and health and safety reasons, all exposed jewellery is banned. Thems the rules. Take it or leave. Literally. No grey areas for "I've just got my ears pierced" either. No jewellery.
Get parts of your body which are covered by workwear pierced to your heart's content. I do not need to know about your Prince Albert piercing (do not google that at work). I also strongly doubt any earring or eye brow ring is "non removable" in any case. What do they do? Weld it? If (somehow) it is, change it for a removable one and replace it after your shift. Done.
