The zebibah, Arabic for raisin, is a dark circle of callused skin, or in some cases a protruding bump, between the hairline and the eyebrows. It emerges on the spot where worshipers press their foreheads into the ground during their daily prayers.
Here is a fine article in the NY Times on the ZEBIBAH.
One of our editors, Ben Barankin, published a letter on prayer marks in the International Journal of Dermatology. Barankin Letter
Dr. Barankin's letter was a comment on:
Abanmi AA, et. al.
Int J Dermatol. 2002 Jul;41(7):411-4.
Dermatology Department, Research Center,
Riyadh Al Kharj Armed Forces Hospital