What are stamps in Lucid Adventure? This is a reward system in place that provides you with Diamonds. Each time you are prompted to watch an Ad, you'll be rewarded with a stamp automatically after you finish watching. You can watch Ads by pressing the bouncing chest at the main menu, or by watching ads through Recruitment, the daily mail, and the in-game Shop.
Filling up your Stamp sheet will provide you will 500 Diamonds, and this gets refreshed every time you fill it up. This is a good way of earning diamonds for free.