A pearl, according to the OED, is "the lustrous accretion formed in certain mollusks," which generally looks something like this:
Interestingly, that is not the first definition in the OED. At the top of the page for "pearl," OED lists: "Early senses in non-literal use, relating to the eye... Any white lesion of the eye, esp. a corneal opacity or a cataract."
So Ariel's association of pearl's with eyes was not original, but instead tapped into a common turn of phrase. If eyes turning to pearls colloquially meant the growing of cataracts, Ariel's metaphor brings to bare the natural progression of living things from sight to blindness. This is interesting when held against the commercial value the eyes hold once transformed to pearl: the transformation becomes one of personal value, i.e. sight, to trade value. In death, Alonso loses his personal status and becomes a commodity, incapable of personal sight.
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