One way to solve this might be to muddy the lines a little.
For example, a level 10 region might give you a 50% chance of getting a tier 2 gem and a 50% chance of getting a tier 1 gem. Level 11: 60/40, level 12: 70/30, 13: 80/20, 14:90/10, 15:100; then level 16 might have a 95% chance of giving you a tier 2, and a 5% chance of a tier 3. Level 17: 90/10, 18: 85/15 and 19: 80/20.
Or, you know, whatever level gates for whatever probabilities you like. It'd still be important to have a dead cert level for each gem tier (in this case 15, 25, 35, etc. - but it could stay like it is at 10, 20, 30 and you just add the probabilities in between) but now there's incentives both ways - go higher and you'll definitely either get what you're after, or maybe a shot at something even better. Go lower and it'll be easier but you might get the tier below instead.
Does that hold any water?