After camping the 3 different locations to find this mob (upstairs dining cook area, basement stalls jester area, and across stalls area in skeleton finger/cracklin bones areas), I believe that you get the sapphire from the thick boned skeleton in the skeleton finger/cracklin bones area.
After numerous kills on the other locations, jester area gave only the thick skull, dining room gave only emerald, and after hours the skeleton finger TBS gave the sapphire.
Get ready for a long camp. Good Luck.