[Alneyan]

Could it be that the pearl increasing the mage level is counted in the "total number of gems a mage can use"? The first pearl your mage used would bring him to level 10, allowing him to use 10 pearls, so two extra. Wouldn't this allow him to (barely) cast the spell?

I am not sure it is intended to work that way, but it seems the most likely explanation to me; that is, if I got my maths right.