I'd created a diagram before, but I lost it.
It's basically something like this. You get 1 randomly every week.
GB ....GB................................GB............................
Day ....1...2...3...4...5...6...7....1...2...3...4...5...6...7
So you see, you are getting the gem every 7 days... but you could get lucky and get this...
GB ..................................GB...GB...........................
Day ....1...2...3...4...5...6...7....1...2...3...4...5...6...7
Or unlucky and get this...
GB ....GB.................................................. ..............GB
Day ....1...2...3...4...5...6...7....1...2...3...4...5...6. ..7
So sometimes it feel like you havent gotten your gem box in a while, then sometimes you get a gem box within 2 consecutive days. I don't think I've heard of a case or if it's even possible to get it within 3 consecutive days.
I think this is as close to the core algorithm as you can get without seeing the code or being told directly by Supercell.

