Mussel shell varies with a range from 135 to 223 Vickers hardness that parallels the drilling results. Bironite has a hardness of 190 Vickers. Hence in a string of mussel shell pearls, some will wear at a faster rate than others will. The Bironite string will wear uniformly. In practise both nuclei will be acceptable, as it is the string itself that is the weak link in the pearl strand.
Jewellers recommend that pearls be restrung regularly.