To get the highest number of points possible, you should aim to clear off the biggest possible combination of diamonds at one time. Aim for the largest section of diamonds of the same color. For example, if you get only 3 diamonds, you earn a measly 100 points, whereas a block of 4 diamonds will give you 400 points. Points can run into the thousands when you clear more than six diamonds.
The bigger your combo of cleared diamond blocks, the faster the bar at the top of the screen begins to fill. This will eventually reward your with a diamond bomb that clears two rows of diamonds, as well as well over 10,000 points per use.
Make sure you try to avoid clearing blocks of two or less diamonds, as this drains your meter.