You get special gems for doing fast speed chains. Special gems are tiles that spawn into the game and you will recognize them from being different colored than the others. You need to connect the two special gems that appear in the game to collect the bonus. The special gems give bonuses such as additional seconds to the game clock, remove all tiles of one kind from the field, turn all of the tiles into the same color so you can go on clicking-spree and click on everything and they will match and so on.
Getting three special tiles isn't hard at all if you just play the Dash mode.