Akoya pearls tend to yellow over time due to the thin nacre layers surrounding it's shell bead nuclei- the more exposure to various chemical agents and time, the faster the pearls will begin to display wear. Gradually the contaminants (i.e. tobacco smoke, perfume and sweat, etc.) begin to affect the crystallized layers and will stain them; Think about the habitual coffee drinker/smoker and their teeth... Less extreme effects in the case of pearls, but definitely there!