| The coefficient of thermal expansion is different. Saltwater nacre and freshwater mussel shell is a good match.
The giant clam beads are bad for akoya. The nacre is thin and as the nucleus expands and contracts it can crack the nacre. This would not be the case with South Sea or Tahitian pearls, but there is another issue with GC beads with those. The pearls tend to crack, or in some cases explode, when they are drilled. |