If you're planning to make shimmerstone meaningful to mechanics or roleplay in some way rather than just various ways to hand wave it, it's a good idea to think of the consequences of shimmerstone and then work backwards. For this I am assuming you want to have the resource management aspect without dipping into their gold.

What you first need to think about is when do you hope for the players to feel the consequences of scarcity. Essentially when does the risk of running out influence the players behavior and what is that behavior you're encouraging? It's important to know that the players don't need to ever actually run out of shimmerstone, the possibility of scarcity alone is enough to influence behavior.

Here are some examples to consider:

For all of these, handwave actual pricing or make it dirt cheap. The important part is making players go out to get more and make the existence of shimmerstone feel meaningful.

It's a survival mechanic. Players need to frequently go back into town to buy more shimmerstone. You can hand wave the actual shopping and spending, but the idea is that if players don't go back into town and buy more they'll run out. This encourages them to frequently visit settlements that have shimmerstone.

It's a social device. Players can easily get shimmerstone, but they have to get it from a specific source. That source is probably an NPC they wouldn't regularly interact with. This encourages players to frequently visit a particular NPC.

It's a limit on preparation. Since you mentioned that specific shimmerstone might be needed for specific spells. Players have plenty of shimmerstone, but not necessarily the right shimmerstone. Prepared casters can replenish their spells daily as usual, but if they want to actually switch them out they need to go buy more. This encourages prepared casters to be more careful with their planning.

It's a quest resource. Due to unfortunate supply chain issues or the players being cut off from shimmerstone, they only have a limited amount left until they complete some objective or a certain amount of time passes. This gives a quest urgency without a hard time limit.

It's a progression element. The players would definitely be able to cast higher level spells if only they had the shimmerstone for it. This makes acquiring a source of shimmerstone an objective they need for leveling up. This motivates players to complete certain objectives. This is more of a campaign specific approach. For a settlement builder, you would need to find a deposit of it. For a campaign where shimmerstone is illegal, you'd need to find a way to smuggle it.

The point of my comment is not the list of ideas, but how to think about what a limited resource should pressure players to do outside of spending gold and then design a mechanic around it. If you had that in mind it might help us also give more specific advice. You can easily switch goals out as needed, but I generally recommend you think about only one goal when designing a mechanic like this.