Why would a pregnant woman have a craving to eat coal?

I know of two women who had this craving and sucked on a chunk of coal.
I want to know the science behind this, I’m guessing carbon deficiency or toxin build up or something but Ive never heard of carbon supplements being recommended for health.

I had a look and the ingredients of coal are aolinite, pyrite, marcasite, quartz, chalcopyrite, galena, sphalerite, and the element carbon. Out of these I only know of carbon.
Anybody have more info on this health issue?