A friend of mine did a little Google prying and found the origin of my grandfather's tie pin!
Wiki's got the answer:
Phi Beta Pi Medical Fraternity
Founded March 10, 1891, at the West Pennsylvania Medical College (now a department of the University of Pittsburgh) as an anti-fraternity society. It was originally known as Pi Beta Phi but changed its name because of an existing sorority with the same name.
Badge is a diamond of gold with emerald points and pearl edges. A black enamel center with gold skull and pelvis and the letters "ΦBΠ."
My grandfather was an OBGYN so it makes complete sense for him to have been in a Medical Fraternity. :)
Not quite as mysterious as an Ivy League secret society as I had first conjectured, but still a neat piece of history.