-V- Loki -V-
... Magma ~ Yes. But it could be in various forms of heat and viscosity, like a hardened fire-shield or as you stated. A character that simply leaked magma would be effective as well.
Smoke ~ A character skilled with smoke magic could turn into smoke and even just portions of their body to avoid strikes and blows. Like the wind elemental from Riddik[sp?] 2.
Ooze ~ Essentially yes, but in mass amounts and likely with some other attribute attached to it.
Vacuum ~ The way I incorporated this into Loki's abilities was positively my favorite. He used it more as a gravity magic. There were these bomb-like objects he could construct with his energy. At the center would be a black orb that emitted it's own gravitational pull, very strong, and surrounding it were six tornadoe-ish spirals that gave off their own gravitational pull. The result being once it would come in contact with the enemy, it would crush their limbs and weapons etc. by catastrophic forces of being plled in both directions. Essentially an arm would be split so each individual palate of muscle and tissue and vane would separate from one another, twist, then break away into void. There are other ways it could be used as well. Like you could surround a sword with a sheathe of vacuum energy to make a strike that could cripple armor.
Radiance ~ The composing elements of positive energy and fire would mean that the rays of light would burn like fire, and at a multiple of that intensity to negative forces, like the undead. Light can blind, that is a useful effect in and of itself.
Ash ~ Negative energy and fire would actually be very powerful. If a character released a field full of ash that not only burned but inflicted pure pain [negative energy being not evil, but anti-life] you would have no where to run, and throughout the battle would be suffering damage.
Steam ~ A character that uses water would have proficiency in this as well as ice. The three forms of water solid, liquid, and gas. They could pass from a dangerous sharp-edged ice creature, a liquid form that could escape through waterways or in battle as a goo-creature, and resort to a gaseous form throughout a battle to avoid fatal blows.
Salt ~ I never got that one either. XD
Mineral ~ This could be used the same way that Kore uses the power of mother earth, Mist. By particles of energy that would speed up the growth of plants and plant-type weaponry. It could also be used to buff up a weak spellcasting type character.
Dust ~ Pure destructive energy. Negative power backed by the strength of earth. Matter would be turned to dust. A thrown blade could be dismantled into the atoms of its creation until it resembles nothing like it was but dust of what it was made of.
Crystal ~ Seems more to me like it should be ice and mineral. Loki used this to create spike bomb projectiles that when explode would release a cloud of sharp crystal shards that would rip whatever was in their range to shreds. He would also implant spikes of the stuff into the ground, which would then explode when someone neared them., they could even be placed underground.
Frost ~ Given the negative it would have a deathly chill to it that ice wouldn't. Say you encased an opponents wrist in ice. The chill would spread up their arm, target their heart and kill the individual in a matter of turns.
Clay and Silt ~ Like Salt, we'd have to ask Kraun.
Obsidian ~ But like magma it could be used as armor. Crystal strategies would work the same. A player with Magma abilities would find it useful to turn structures they have created to a hard glass like substance, to the extreme of creating an arena all their own, that they can manipulate by generating more magma and hardening it.
Pumice ~ A typical attack of Pumice would be an explosion or blast attack that releases millions of sharp particles into the air like small pieces of rough sandpaper to mercilessly tear your opponent to shreds.
Sparks ~ I never figured how he came up with that from positive and smoke.
Fumes ~ Yes. But the user could emit various forms of smoke with various effects. ...