Subject: What is the process that creates a stone baby?

Hi, I recently came across this website [Moderator's Warning: The
following site contains graphic pictures that may not be suitable for 
children and the sqeamish - ADULT SUPERVISION ADVISED]
http://www.obgyn.net/ENGLISH/PUBS/ARTICLES/Stone_Baby.htm and it begs the 
question, what exactly the process that creates a stone baby? Is it the same as 
an oyster creating a pearl? Why wouldn't a non-viable fetus simply be 
reabsorbed into the body since it's made of the same material as the mother? 
Why calcium and not a fluid buffer? Why did this happen to a baby and doesn't 
happen to tumors or a bullet that can't be removed? Is there a biological 
advantage to this? I'm sorry there are so many questions but this is just too 
fascinating.

Re: What is the process that creates a stone baby?