“All the calico scallops I’ve ever seen are brilliant
colors on a white base. While recently visiting Apalachicola and the
state park beaches on the neighboring island, I found the calico
scallops were black. The white base was black or gray and the the
brilliant colors had black in them as well. I noticed this of other
shells in that area. Shells that are typically white everywhere else,
were black: nutmegs, lettered olives, figsnails, venus clams to mention
a few. The beach sand was not black nor was there any blackness among
the white sand. Can you tell me what had caused this? They are strange
looking but makes for an interesting shell collection.

Quickly, I got this reply:

“My professional guess would be that these shells were washed up from an
offshore area where they were buried in anoxic mud. The hydrogen
sulfide in the mud would turn the shells black. There are other
possible explanations, including that all of the animals live in an area
where shell construction involves the incorporation of a black pigment
from some unknown source, but the fact that shells from so many
different species are similarly colored suggests to me that it is a
secondary exposure. If [you] can break or cut a few shells [you] could
determine if the black coloration is a surface feature or if it
penetrates the entire shell and is therefore characteristic of the life
of the shell. That would aid in discriminating between options.”

Well, I did break a few shells and found the color to penetrate right through so I’m guessing the shells grew in an area where the sand pigment was dark.  Same with the orange shells.  Something in the sand, whereever these particular shells grew, gave them that color.  How cool.