Hi,
Both the irradiation and dying techniques are stable and the results do not change over time. Irradiation does not leave residual radioactivity and is perfectly safe. The process is also very rare because the necessary equipment is very expensive. Most black akoyas are dyed. However, I recently found evidence that there was some culturing in pinctada nigra going on and has been for about 12 years. These would be naturally black. Also, pearls growing near the liver will turn out naturally black. There is no non-destructive way of telling which is which. As such, the general assumption is that all black akoyas are dyed, which they are clearly not, because testing would cost far more than the product.
Zeide