How to get free diamonds?

There is not a way of getting free diamonds, but you can get diamonds by:

- Sharing (2-5)

- Completing spirits of sheen

- Winning in competition or stylist's arena (10-100 & 5-400)

- Chance of getting liking photos in starry corridor (2-5)

- Chance of getting in daily lucky draw (2-4)

- Using diamonds in the fortune cat

- Sometimes signing in (50-150)

- Completing archivements or suits

- Completing quests (65 diamonds)

Other than completing in-game achievements, quests, etc., there is no real way to obtain "free" diamonds. But, a way out of game to earn money (via completing surveys) is Google Opinion Rewards. The only thing you have to do is complete surveys to earn money you can use on Google Play and in games. I've used this service before to obtain diamonds in Love Nikki, so I can confirm that using Google Play Balance gained from the app does work. Hope this helps!