Do all spells that require a focus need their own dedicated focus, or can you use the same item as a focus for multiple spells as long as it meets the criteria? For example, say you have three spells that as foci, respectively, a gem worth 10gp, a gem worth 50gp, and a clear gem or prism. Would you need to have all those in your inventory, or could you just use one clear gem of 50gp for all three spells?
There are no general mechanics tying an item used as a focus to a spell after it has been used.
In the case of gems, just make sure the GP values match what each individual spell demands.