Dark Variety Lettered Olive

I found these lettered olives (Oliva sayana) on the beach today. It was unusual finding an olive this large, but the dark color... love it.

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Sally Thomas: Read Orrin Pilkey's How to Read a NC Beach, Chapter 6 pg 113 on the subject of brown and black shells. These are secondary colorations and a result of the reaction of iron to either oxygen (brown or rust of iron oxide) or sulfur (iron sulfide). This may be your answer. Good luck, Sally

Sharon: My dark olives appear to be fading to a lighter color, as they've sat in their box.