The Skull Gem is one of the 3 unused (BETA) gems in the game.
The Skull Gem after a move will eat the Gems Below it (mostly like 1 each move) when they reach the bottom of the board... The Game will be lost and CRASH. BETA Options said that these gems will go to ALL modes after Classic level 23, But they are ignored by the Game.
They appear as a blank space on the board, But if you hover your mouse cursor in it it's color will be revealed in glow.
Removed by Unknown Reasons, the gems can still be in the game after Editing Save Files