I decided to use a special stone for multiple spellcasting. I called it singularity (on Numpad 1 per default).
A singularity has two main uses.
In case you just used a spell, you can cast it again simply by using a singularity.
When spellcasting, some strings can be replaced by singularities.
For example, if you need to type three fire gems, instead of FFF, you can do F1. The result is the same once the spell is cast.
The cost in singularities for these uses is very straightforward: one singularity each time you use it. Except for re-casting a spell where half the length of the spell is required.
A singularity costs 150 gems fragments, so it stays a rare resource.
For everyone to be able to test it, 100 singularities will be available on your first login after the update.
The update will come out as soon as this week as long as there is no complication.

By the way, a little gift on my part, the estimated cost for spellcasting repeat is rounded to the floor (unless it is less than one to avoid fireball spamming).