Login rewards are mostly random. Sometimes you get 10 Gems in 10 days, another round you got no Gems in sight. Last few rounds for me have sucked but it is getting better with me getting 10 Gems again. I am still waiting when there is eventually 5 Gems per day bonus(DMM version got this). I think there is also a 100+ Gems gift for how long you played as well(based DMM version), so hopefully we get it in the future

According to DMM version, you get a free Rainbow Alstroemeria for exactly one year of playing. Not sure if you get another free Rainbow in other occasions.