Hi Xeresana,
I personally have no Fireballs, but if one looks at the tail, that would indicate rather thick nacre above the nucleus. My guess is perhaps more than 1 mm on the radius, a total of 2 mm.
Of course, this could be changing very fast, as the farmers in China surely are out to make a fast buck, so perhaps in a couple of years it goes the same way as with the Akoyas. That would be a pity though!
I hope some of our experts can tell us more........