Skull Gem is a type of gem that was scrapped from Bejeweled Twist. It has no graphics and instead appears as a bare space on the board. Using a hex editor, you can make Skull Gems appear on the board. 
Through a hex edit modification to the game's files, it is revealed that when a Skull appears, it continuously chomps and destroys the gems below it without regard of moves. The game is over when it reaches the bottom of the board. Thus, the player is required to actively "juggle" the Skull and to destroy it with Special Gems in a quick manner to end the threat.
In the Spanish version of Bejeweled Twist, Skull Gems and Doom Gems share the same name.
According to evidence found inside the game files, it appears the Skull Gem would have appeared after Level 24 in Classic Mode. If this setting is changed, the game will freeze before it gets to the main menu.
If the Skull Gem reaches the bottom of the board, the game ends and the game crashes.
The message when the Skull Gem first appears shows "What's this? This SKULL is chomping thru your gems! Destroy it before it hits the bottom!" and "If the Skull reaches the bottom, it will destroy one of your Shards!"
It's possible that "Shards" is a scrapped mechanic, possibly for a mini game.
Skull Gems functionally resemble Fire Tiles from Bookworm Deluxe. In Bookworm, every time a word is spelled any Fire Tiles in play destroy the tile below them. If a Fire Tile reaches the bottom of the screen in Bookworm it's game over.