Complex sentence: The game is over when the last gemstone is placed, all the tiles are covered and only the four exception gems remain in the bag then, by comparing accrued points totals, the player with the most points is the winner of the game.
Split into simpler sentences: The game is over when the last gemstone is placed, all the tiles on the board are covered and only the four exception gems remain in the bag. Then, by comparing accrued points totals, the player with the most points is the winner of the game.