Signal groups of at least 3 gems that are coincident with the center of each of the 3 areas to eliminate them and earn points. Remember that they must be coincident with the central color, that is always changing. If more gems of the same color are close to the 3 you marked they will also disappear and you'll earn more points. As you earn points, small white diamonds extend at the right side, from the top to the bottom; when they reach it you pass to the next level. The time is shown at the bottom left corner of the board, as red dots: when they reach the ground, you lost. There's also gems that change their color or are multicolored, so they can be used as a joker and grouped with any other.
Click on the group of gems whose color you want to change. Keep an eye to the color of the 3 central gems, because if they have the same color, a click at any of them will give you a lot of extra points.