Observing and testing the ideas and examples in this thread so far have produced mostly just more questions, but I've found a few things out too.
Ice spike spell without custom material:
[CAN_DO_INTERACTION:MATERIAL_EMISSION]
[CDI:ADV_NAME:Cast Icebolt Spell]
[CDI:USAGE_HINT:ATTACK]
[CDI:BP_REQUIRED:BY_CATEGORY:HAND]
[CDI:MATERIAL:WATER:SOLID_GLOB]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:15]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:VERB:conjure a bolt of ice:conjures a bolt of ice:NA]
[CDI:WAIT_PERIOD:30]
The line "[CDI:MATERIAL:WATER:SOLID_GLOB]" is curious, but gets the effect done. SOLID_GLOB apparently auto-adjusts whatever material you're trying to use by setting it to whatever temperature it needs to be to become solid. In this case, it finds the game's existing water material and freezes it to make it SOLID. LIQUID_GLOB, conversely, appears to melt most substances, so you end up flinging, for example, molten iron if you're using INORGANIC:IRON.
Regardless, the temperature of the material doesn't actually seem to matter, so every one of these projectile material emissions behaves like a thrown object. I have yet to see a stick-in of these thrown materials.
It is also noteworthy that SOLID_GLOB and LIQUID_GLOB material emissions rely upon the throwing skill for effectiveness and accuracy.
Summon water spell, for thirsty adventurers:
[CAN_DO_INTERACTION:MATERIAL_EMISSION]
[CDI:ADV_NAME:Cast Fountain Spell]
[CDI:USAGE_HINT:ATTACK]
[CDI:BP_REQUIRED:BY_CATEGORY:HAND]
[CDI:MATERIAL:WATER:SPATTER_LIQUID]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:15]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:VERB:conjure a tiny fountain of water:conjures a tiny fountain of water:NA]
[CDI:WAIT_PERIOD:100]
Stand on top of the water it makes, and you can drink or fill your waterskin. The moment you walk away from the water square, it is smeared across the ground and evaporates.
SPATTER_LIQUID effect appears useless for temperature-based offensive purposes. The feet of an enemy will be coated with whatever (usually molten) material you use, like granite for example which coats the target in magma, but there never seem to be any burns as a result.
I haven't tested syndrome application with this delivery system yet, but that is likely the only use of this emission method for right now.
Steelskin spell, for adventurers with fire/high temperature resistant bodies:
[CAN_DO_INTERACTION:MATERIAL_EMISSION]
[CDI:ADV_NAME:Cast Steelskin Spell]
[CDI:USAGE_HINT:ATTACK]
[CDI:BP_REQUIRED:BY_CATEGORY:HAND]
[CDI:MATERIAL:INORGANIC:STEEL:TRAILING_VAPOR_FLOW]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:15]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:VERB:conjure steel vapor:conjures steel vapor:NA]
[CDI:WAIT_PERIOD:1000]
I tested this effect first as an offensive ability. It does indeed cause heat damage by coating targets in molten steel via enveloping them in a cloud of it if they are within the (very random) range.
I noticed, however, that my character was also coated in steel with every use, thus forcing me to modify my race with these tags so I didn't melt:
[HEATDAM_POINT:NONE]
[IGNITE_POINT:NONE]
[IF_EXISTS_SET_MELTING_POINT:NONE]
[IF_EXISTS_SET_BOILING_POINT:NONE]
Being immune, every use coats me with steel. I'm not 100% positive, but during regular combat, my entire body coated with steel appears to help deflect blows. I have seen weapon attacks "glance away" very rarely while otherwise naked but coated in steel, whereas before they did not.
This could very likely be an observation error on my part though, since I haven't tested it too extensively.
On the other hand, transforming yourself into a human-sized steel colossus definitely bestows the defensive properties of the metal properly:
interaction itself:
[CAN_DO_INTERACTION:START:0]
[CDI:ADV_NAME:Cast Steelbody Spell]
[CDI:INTERACTION:STEELBODY]
[CDI:TARGET:A:SELF_ONLY]
[CDI:USAGE_HINT:FLEEING]
[CDI:BP_REQUIRED:BY_CATEGORY:HAND]
[CDI:VERB:transform into steel:transforms into steel:NA]
[CDI:WAIT_PERIOD:6000]
interaction_wizard.txt, required for interaction to work:
interaction_wizard
[OBJECT:INTERACTION]
[INTERACTION:STEELBODY]
[I_SOURCE:CREATURE_ACTION]
[I_TARGET:A:CREATURE]
[IT_LOCATION:CONTEXT_CREATURE]
[IT_AFFECTED_CLASS:WERECURSE]
[I_EFFECT:ADD_SYNDROME]
[IE_TARGET:A]
[IE_IMMEDIATE]
[SYNDROME]
[CE_BODY_TRANSFORMATION:START:0:END:20]
[CE:CREATURE:STEELSKIN_WIZARD:DEFAULT]
Creature for interaction:
[CREATURE:STEELSKIN_WIZARD]
[DESCRIPTION:A wizard whose skin has magically turned into steel.]
[NAME:steelskin wizard:steelskin wizards:steelskin wizard]
[CASTE_NAME:steelskin wizard:steelskin wizards:steelskin wizard]
[CREATURE_TILE:'@'][COLOR:7:7:1]
[THICKWEB][WEBIMMUNE]
[FIREIMMUNE_SUPER]
[NOPAIN][EXTRAVISION][NOBREATHE][NOSTUN][NONAUSEA][NOEMOTION]
[NOTHOUGHT][NOEXERT]
[NO_DIZZINESS]
[NO_FEVERS]
[NO_DRINK][NO_EAT][NO_SLEEP]
[NOT_LIVING]
[CANOPENDOORS]
[NOT_BUTCHERABLE]
[EQUIPS]
[NOFEAR]
[PREFSTRING:toughness]
[NOBONES]
[BODY:HUMANOID:2EYES:2EARS:NOSE:HUMANOID_JOINTS:5FINGERS:5TOES]
[NO_THOUGHT_CENTER_FOR_MOVEMENT]
[TISSUE:STEEL]
[TISSUE_NAME:steel:steel]
[TISSUE_MATERIAL:INORGANIC:STEEL]
[MUSCULAR]
[FUNCTIONAL]
[STRUCTURAL]
[RELATIVE_THICKNESS:1]
[CONNECTS]
[TISSUE_SHAPE:LAYER]
[TISSUE_LAYER:BY_CATEGORY:ALL:STEEL]
[BODY_SIZE:0:0:70000]
[BODY_APPEARANCE_MODIFIER:HEIGHT:75:95:98:100:102:105:125]
[BODY_APPEARANCE_MODIFIER:BROADNESS:75:95:98:100:102:105:125]
[ATTACK:PUNCH:BODYPART:BY_TYPE:GRASP]
[ATTACK_SKILL:GRASP_STRIKE]
[ATTACK_VERB:punch:punches]
[ATTACK_CONTACT_PERC:100]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:MAIN]
[ATTACK:KICK:BODYPART:BY_TYPE:STANCE]
[ATTACK_SKILL:STANCE_STRIKE]
[ATTACK_VERB:kick:kicks]
[ATTACK_CONTACT_PERC:100]
[ATTACK_FLAG_WITH]
[ATTACK_PRIORITY:SECOND]
[ITEMCORPSE:STATUE:NO_SUBTYPE:INORGANIC:STEEL]
[ITEMCORPSE_QUALITY:5]
[DIURNAL]
[SWIMS_LEARNED][SWIM_SPEED:2500]
[SELECT_MATERIAL:ALL]
[COLDDAM_POINT:NONE]
[HEATDAM_POINT:NONE]
[IGNITE_POINT:NONE]
[IF_EXISTS_SET_MELTING_POINT:NONE]
[IF_EXISTS_SET_BOILING_POINT:NONE]
Keeping the user's spellcasting abilities is as simple as putting the same interactions from your default race into the steelskin wizard creature race as well, excepting the steelbody spell of course.
It's quite a pity that all equipment is removed by transformations at the moment, as I originally had this spell configured to be a stoneskin spell, but stone tends to be too brittle to be helpful while you're naked. Steel is much more resilient, but you will still get shattered, dented, and dismembered all too easily by enemies with higher stats and better weapons, since you're naked.
Sticky magical metal spellsilk webbing spell:
[CAN_DO_INTERACTION:MATERIAL_EMISSION]
[CDI:ADV_NAME:Cast Silverweb Spell]
[CDI:USAGE_HINT:ATTACK]
[CDI:BP_REQUIRED:BY_CATEGORY:HAND]
[CDI:MATERIAL:INORGANIC:SILVER:WEB_SPRAY]
[CDI:TARGET:C:LINE_OF_SIGHT]
[CDI:TARGET_RANGE:C:15]
[CDI:VERB:conjure streams of spellsilk:conjures streams of spellsilk:NA]
[CDI:MAX_TARGET_NUMBER:C:1]
[CDI:WAIT_PERIOD:200]
This one is just the giant cave spider's web slinging ability that paralyzes the target and makes sticky mines on the ground that paralyze whoever walks into them, temporarily. Like the other preset emissions (fireball, firejet, dragonfire), this emission method conveniently casts its material in the direction you aim without hitting the user on the user's square upon casting. However, you can still get stuck by walking into a sticky mine, unless you give your race the [WEBIMMUNE] tag.
Interestingly, the material can be anything, and still has the webbing effect, and the sticky mines produced are "[material] thread". In the above case, silver thread is produced.
You could probably produce ice thread and other such things, for variations on an elemental theme. Silver thread is melted into molten silver if heat is applied to it, by the way. Food for thought, with other materials that are forced into a solid state.
What I would really love to see that I haven't figured out yet is making even better use of existing vanilla materials modified by interactions to achieve desired results.
For example, can we find a way to produce vanilla, ingame snow from interactions? Is there any raw reference to snow? Is snow just a particular temperature of water?
Since we can make ice from vanilla water via interactions, what else can we do with water? Can we create depths of water and actual waterflow? Can we cool water enough to cause freezing damage without making the water into solid ice, and likewise can we cause damage from boiling water, consistently and usefully, without creating new materials?
If not, how do we base new materials on existing water? Water doesn't seem to have any presence in the raws either.
I'll be looking for answers to questions like these, but there might be no answers in the current DF version. If someone else figures this kind of stuff out, please feel free to say so here.
(EDIT: VAPOR_FLOW emissions have interesting effects on all objects in the world that they hit. Earlier, I coated the ground in silver thread from my silverweb spell, and then blasted the thread with molten steel from a TRAILING_VAPOR_FLOW. The result was molten silver laced with molten steel, and a very high temperature in present and adjacent tiles. GAS_FLOW emissions do not seem to properly apply temperature effects, though, only syndromes as far as I can tell.)