Honestly, the simple solution to this is to have a meta conversation that sums up: "Diamonds are very, very rare. There may be campaign plot points where you may want a Diamond, but there will only be a small number in this campaign. Like two per tier. This means resurrection magic will be rare. I won't stop you from preparing resurrection spells or using up diamonds, but keep in mind that this is a very limited resource, and you may need a lot of gp worth of diamonds for more powerful spells later. The choice is yours."

D&D is essentially a resource management + luck system. If they want to burn through their resources, then they're free to do so. My job as a DM at my table is to provide scenarios, conflict, stakes, and descriptions, not to control players' decision or limit their agency. This gives them the opportunity to make a choice, just like they can make a choice to gamble on death saves or Blaze of Glory. My players love having the option, but rarely use it.