He was never explained properly I think ? The most actual information we got is when he demontrated his power on his ' Legionaire interview'. Something like he can control both Fire & Water simultaneously.
It's complicated and explained much better in the novel.. but to put it simply..
Yuder is capable in manipulating or controlling any nature —you name it; water, fire, air, land, etc.—
That includes anything that is made and a product from 'nature' or from natural origins.
So yeah, his ability is nature based. Fork is made from land minerals/materials
Why did he took a fork from the table? XD
I forgot, his power was using the 5 elements, right?