A Devil’s Inkwell is a ball of sand that has been petrified from iron oxides,& at some time a chip of it is broken off, the sand on the inside is washed out by wave action leaving a hollow ball. This piece was found in Aquinnah, or Gay Head. A perfect heart was chipped off naturally. Then I inserted an image of the Gay Head clay cliffs inside the Heart.
It measures about 5.5 inches long & 6 inches round.