Wyl, as you already know, is an Earth Elementalist.
This means he can manipulate, use, and conjure anything (including plant life) of the Earth, and also allows him to 'mindlink' to creatures and other such entities that are Born of the Earth.
He favors using Diamond, mainly because of its magical properties and natural hardness, but can and will use any other earthen mineral. The other reason he uses Diamond is; that when he gained his complete Mastery, he also gained the ability to change his body, and to some extent his mind, into that of a Diamond Elemental.
The Diamond Elemental is some 14 foot tall, and made purely of Diamond. This transformation is extremely painful and takes several minutes as his entire body must grow and completely change its structure.
His abilities make him a gifted smith, but not as good as some. The ability to summon a pure bar of whatever metal you wish to forge is a highly useful thing.
He can also do the usual 'throwing' rocks around, levitating them, and small things like that. Usually in a fight or confrontation he will lob the occasional rock, but a more favored strategy is to fling several needle-sharp shards of a gem or metal at the target. He also lines his fists and arms in either Diamond, Steel or Granite.
Defensively, Wyl's element is exceptionally useful. A commonly used tactic is to either form a bubble of Diamond around him which protects him from most attacks, or to sink into the ground and either lurk underground or emerge away/behind from the aggressor. He can use several other methods, but has yet to.
His Water abilities are nowhere near as potent as his Earth, though the natural healing power that comes with the knowledge of Water is priceless to him. He can also siphon water from a nearby body, and use it in a variety of different ways.