Showing or telling?

A quick example, Ora:

He was cold is telling.

He tightened his scarf and turned his face away from the biting wind is showing.

Now, to your sample:

Ahbel regarded him for a moment. [that's telling; you're in Ahbel's point of view, so if you describe him thinking the following, you don't need to say he regarded him; we'll know]

So I would simply render it this way:

Remarkable how quickly his striking face could go from appearing [young can't unequivocally say lighthearted, because you're in Ahmed's head, not Eliyah's] lighthearted to serious [you don't also need and contemplative; again, he can only guess at contemplative]. Eliyah got the feeling [whoops! no, this is a point of view violation; you have hopped from one head to the other in the middle of a scene, in fact in the middle of a paragraph; you get one perspective character per scene, and if and when you shift, you need an extra space between paragraphs and crystal clear wording] that a mighty intellect hid behind the guise of gentle old man.

[In fact, now that I study your sample more, I may have confused the two characters; are we in Eliyah's perspective? if so, Ahbel can only appear to be regarding him, and you need to be clear whose striking face we're talking about; a communication breakdown like this is always the fault of the writer, not the reader; it's our job as writers to make things clear]