The stacking of the gems!
Columns is an shuffling and arranging kind of puzzle, where your main goal is to stack the falling blocks of gems in such a manner as to have a continuous set of at least 4 gems of the same color, either sideways, vertically or diagonally. At times you get special gem blocks, which, as in Tetris, fall at random from the upper portion of the screen. These have different properties, but the idea is that they generally can cut through your blocks, and if you arrange them smartly, depending on the gem configuration where they fall, you can make more than just one block of gems disappear. Each level of the game has a different speed and also, the later on have more special blocks falling, but the speed of reaction you show is also much more important. So, similarly, again, to Tetris, you play for points, until your towering block of gems has reached such heights that the game is lots. It's very involving and very comforting game, yet also a pretty straightforward game, in terms of the way it is built. Have it in your puzzler time waster collection, though!