Per the pink and silver undertones, I believe you're referring to the body color. This same body color can be found in other types of pearls and doesn't necessarily indicate the pearls are South Sea. It's quite common in freshwater pearls too.
What you're seeing with the akoya is a
thin-film interference effect. The light reflects from the surface of the pearl, and penetrates and reflects from inner layers. This overlapping strengthens and weakens the light waves, which creates the colorful optical illusion (orient) you often see in akoya pearls with good nacre. It's similar to what happens with soap bubbles when light reflects from the front and back surfaces, interfering with each other.
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