Sarah,
I believe “Geronimo” is the shop mark… I will do some research and see if I can find another similar example. Also, I believe Jason replied to a separate thread in which he explains in detail the Geronimo shop mark. Try searching the forum for “Geronimo.”
EDIT: Here is a link to a different thread on Turquoise People in which Jason describes some details about the Geronimo mark. P.S. that is a beautiful rock right there!!!