I think you are very fortunate to have found it. It certainly could be considered a pearl, but it's kind of a "franken-pearl". Gemologists tend to call these non-nacreous (not pearly) pearls "calcareous concretions". It probably doesn't have a lot of value other than as a keepsake to remember a cool find. It's very rare to find something so unusual while eating an oyster. This is the first time I have heard of someone finding a two-toned pearl while eating an oyster. Quahog clams have white and purple and they often have two tones.