Paua shell is basically different forms of calcium and other minerals.
Calcium varies in appearance from a seashell, cement, concrete, plaster of paris, teeth, enamel toilet..garden line, wall board , the hard water line around a spa/ swimming pool. your bones.

Cacuim is eaten by acid... battery acid H2SO4, oil vitriol, sulphuric acid are all the same thing but can be at different concentrations.
The type and concentration of the acid, length of time, determines how fast the calcium is eaten into.
The white outside is a form that is eaten faster than the pretty part...
There are other acids also
mild like lactic acid.. in milk, acetic acid in vinegar, citric acid in citrus fruit and the packet in the spice cupboard, toilet bowl cleaner (hydrochloric acid)

All will do the same thing to a shell, at different concentrations, over time frames.

Once the shell is stripped of the white deposits, a buff up on a buffing pad on the bench grinder, with a fine cutting rouge.. then a fine seal coat of a clear lacquer

The bench grinder wire wheel and dust mask is by far the best to clean off, and a buff with a pad and rouge to finish... The down side to this method is the outside often has had some sort of tunnelling 'bug' going thru and to get these out the shell can get rather thin in places.
Where as the acid method cleans out these imperfections better leaving a well presented but 'contoured' outside surface.

I have a whole box of shells was experimenting with a few yrs back for lures and 'eyes' on luminescent painted stray line sinkers.

Hmmm....never tried that , thread, with caution, my chemistry off the of my head says that has potential to maybe get a little 'hairy'... from the reaction one could get depending on the concentrations of the chemicals...maybe the order of which u mix one into the other and gases given off....
Strongly suggest do a lot of homework before going down that route....

Any chance of pics?

ethan18724 wrote: Just get the shell, and put it in a ice cream container with milk, and leave it for 2 weeks. the curdling process of the milk reacts with the outer calcium which breaks it down. after the shell is taken out. just wipe the inside of it with a sponge an ajax. this worked well for me.