Rainbow bands on white South Sea pearl. Why?

[LorenLL]

This is a recent purchase from a department store jewellery fair. According to the salesperson it is a 9mm off-round natural colour white South Sea pearl. The overall body colour is bluish silver-grey, with a ring of rainbow colour bands encircling the flat side. The rainbow bands are on the surface of the pearl but they are not overtones or orient, and it transits from yellow, teal, violet, to a brownish yellow centre, somewhat like the colours of the lip of the pearl oyster shell.

I've seen SSP/Tahitian keshis with colour transitioning from white to gold/black, but never a rainbow band on a pearl. How and why does the band happen? Does the band have any implication on the quality of the pearl?