Lucky Diamonds

The lucky diamonds mechanic works once for every 7 car part purchases with trophies in its respective section, the diamonds accumulate over this time giving you the guaranteed minimum 4 star car once you've accumulated the 7 total diamonds. So the 7 car part purchases made with trophies while accumulating the total 7 diamonds will still have a chance to roll between a 3-5 star car.

Once the bonus is active the diamonds will look like this: