I know it's a necessity with the current design goals of the game but the way spell gem tiers work just isn't fun. Instead of feeling like you're going for a tough fight that will reward you with great strength if you succeed it feels more like a shopping trip where you're just replacing the gear that's getting obsolete again. You fight a (fairly easy) boss and take a gem that just lets you use the spells you already have for another 10 levels before they degrade. It doesn't feel like an upgrade, more like a necessity. When you emerge and use your new spell it performs exactly like the old one.
I don't know what could be done but I know it can't stay the way it is.
I would argue that you are expecting more from the tiers than you should be, they aren't supposed to be a tough fight with a big reward at the end. They are a way of treading water, It is a necessity not an upgrade. they are the functional equivalent of basic quest rewards in MMO's, a slow drip feed of superficial rewards designed to keep you in the game, not ahead of it.
Similarly, If this were say, Diablo, then you would experiencing the same thing, you would be fighting through groups of enemies with slowly increasing stats, making your weapons relatively worse, until *clang* something drops a weapon with higher stats and you are back to parity.
The only real difference here is that this is honest about the lack of actual increase.
Spell gem tiers could be completely removed but that would mean that once you crafted a spell you never need the crafting cost again, which would minimize both the usefulness of gems and the raw materials and the interest factor, past a certain point, which you would reach fairly quickly, materials would be pointless, you would already have sufficient to craft every spell in the game.
As it stands I NEED to go get gems regularly, if i wish to upgrade my meteor spell, I NEED to go into another commodity tower, without the tiers creating demand then caves and towers are as dead to me as buildings are.
keeping in mind that tiers are supposed to be meaningless the other options sort of boil down to:-
visual changes, which wouldn't work, because you'd either have to hand-design the increases (impossible to do for infinite tiers), just add particles as tiers increase (a bad idea for infinite tiers), and it would generally interfere with spell recognition.
Scale the numbers, this is what most games do, so that you feel like you are accomplishing something when you aren't. So for an over simplified example fireball level 2 does 110 instead of 100 damage but everything has 110 health now anyway. but this is more complex to balance than the current system
And I'm done ranting, If you can't tell i quite like the spell tiers.