Normally, you can only charge a spellgem if you know the spell. You can also only learn a spell from a charged spellgem. This can be a bit of a problem if you're holding onto a spell to learn it later and accidentally use it.
I would like to suggest, for that reason (and others), that a spellcaster with Tap Font be able to charge a spellgem of a spell they do not know providing that they're tapped into a font of the appropriate damage type (or possibly gem color too)? This would charge the font as usual, of course.