e.g. by Deep Spellcraft.
First of all, it’s super unintuitive for something that appears in the spelldrop down with all of the choices for modification to be unmodable
Second the spellgem changes mean that wizards who chose a spell that is a common immunity have to pay double the price to cast using spellgem sometimes (ticking between both spellgem/not and eldritch blast) than a caster with a more universally effective element.