A gallstone is a small, hard object, in the shape of a pebble that sometimes forms in the gall bladder or bile duct. These gallstones can block the pancreatic duct if they aggregate near the Ampulla of Vater and will disallow the release of pancreatic enzymes from the pancreas. The pancreatic enzymes will not be able to flow into the small bowel and become backed up in the pancreas. This can lead to pancreatitis.