This is a diamond variation of the Craik O’Brien Cornsweet illusion. What you have to do, is to look at each row of diamonds below. Does each row appear darker than the row above it? It sure does to me. In reality, all of the diamond shapes present in this figure are in fact identical in luminance. The reason for this illusions, is that when combined in this pattern, the diamonds near the bottom are perceived darker than those near the top!
UPDATE: Check out the video below, recently uploaded on YouTube. This guy showed how this illusion works in real life: