"Colored gems fall from the top of the screen in groups of three. Any time three or more adjacent gems of the same color line up vertically, horizontally, or diagonally they will disappear. Any gems above them will then fall down; if these create new groups of three or more gems, those too will disappear, leading to chain reactions. Your job is to score as many reactions and chain reactions as possible before the gems reach the foul line. If any gems remain over the foul line after all reactions are complete, the game will end."
But that's not all. By pressing the Fire button, the gems change positions. This makes up for the fact that the groups of three gems always are vertically aligned, like in Meteos. So, a gem that is on top originally can be changed to the bottom once it lands.
And another thing: the further you get in the game, the gems change shape. As you can see from the screenshots above, this is one heck of a game that leaves me wondering: Why isn't John Payson working for a bigshot game making company like Nintendo, Rare, or Sony?
One of the best, if not the best, homebrew game for the Atari 2600 system so far.