This one is from a fan named Alexandra who
apparently did this on the side of her math notes.

I'll just share Alexandra's explanation for this one:
"My brother's girlfriend came in to teach the class for a day and she does
photography and she was showing us that you can use clam shells and such
to make art by "carving" the inside which shows different shades of colors
depending on the clam type shell. The deeper the cut, the darker the color."
Pretty cool, eh?