Cinder Fall
Silica
Ruby didn't think that "glass" was Cinder's Semblance, but according to the Q&A panel in Germany, apparently she was close enough.
Cinder has the power to manipulate silicate minerals – sand, basically. At least to the extent of fashioning it into glass, after which she has a brief window of time in which she can telekinetically control the glass enough to hurl it at her enemies.
Against Amber and Ozpin, she used super-heated earth or stone as a source from which to fashion a volley of glass shards, which she launched with a wave of her arm. She executed a similar attack against Ruby in the CCT tower with vials of simple sand.
Her black glass swords might be made from special mineral sands conjured from Earth Dust, possibly with some Fire Dust mixed in to help heat the sand to create extra-durable blades.
Merging the swords together and conjuring a flexible string between them might have also been thanks to her ability to control sand; she could alter the structure of the glass constructs.
Cinder's ability to use Fire Dust to turn the ground into a shrieking explosive may be a technique that relies upon this Semblance. Perhaps her ability to annihilate Glynda's stone projectiles comes from the same synergy.
Dust and Magic
Someone once asked me what Cinder Fall's Semblance might be. I wasn't really able to answer them, because Cinder is a deceptively multifaceted character by design; both hers and the writers'.
First, we would need to eliminate all of the abilities she displays which arise from Dust and Fall Seasonal Maiden magic.
Before I go any further, though, I'd like to remind everyone that the Volume 3 DVD Commentary revealed that the Four Seasonal Maidens were not originally a planned part of the story back in Volumes 1 and 2. They were a change made to the background plot before Volume 3 began, by Monty Oum himself before he died.
Yet clearly, this was not something totally different from whatever they originally had planned. The Volume 2 opening featured Pyrrha with an autumn leaf motif, and Cinder Fall had a similar motif as well as a flaming eye very much like the kind we would later see associated with the powers of the Fall Maiden. So, rather than an addition to the plot, the Seasonal Maidens seem like they might be a refinement or expansion of an idea that was already in place.
What this means is that the stolen half of Seasonal Maiden power that Cinder held in the beginning of Volume 3 might have been something different from the source of power that Cinder had in Volumes 1 and 2, which could lead to certain inconsistencies. Or there was no real change at all.
Just something to consider.
Volume 1, Chapter 1 "Ruby Rose"
* The Dust woven into the sleeves of Cinder's dress lights up, and she summons a blast of fire to hurl down upon Glynda; we don't see Cinder's eyes yet.
* Glynda blocks the fireball with a "magic circle"; the remains of the fire strike the ground around and behind Glynda.
* We now see Cinder's eyes, brightly glowing gold in the dark; they remain that way for the whole fight.
* The Dust in Cinder's sleeves is still glowing, and Cinder gestures to command the fire embedded into the ground to rise up as a Screaming Eye Portal that erupts into an explosion of fire.
* Cinder throws more bolts of fire to destroy the debris that Glynda is throwing right back at her.
* When that doesn't work, Cinder pulls more power from her sleeves, creating an array of "magic circles" around her to create what seems to be a "counter-magic / spell-breaker" effect that destroys Glynda's projectiles.
* Cinder blocks Ruby's bullets with her bare hand; her eyes and sleeves are glowing as she does this.
* With a wave of her hand and no previous fire, Cinder creates five Screaming Eye Portals in a half-ring behind Ruby and Glynda. The purpose of the placement is clear when Ruby doesn't even seem to realize what's going on; only Glynda notices, and then has to make sure they're both moving to safety.
Note: "Magic circles" do not reappear after this episode. Was it an early element of the setting which might have been dropped so that the Schnee Semblance would be more distinct, or have we simply not seen anyone need to use one since?
Volume 1, Chapter 18 "Black and White"
* Cinder makes an intimidating show of glowing eyes and a flaming hand to remind Torchwick who is boss.
Volume 2, Chapter 1 "Best Day Ever"
* Torchwick is starting to ask questions about Cinder's operation; she traces Torchwick's jaw with her hand and tells him, "You'll know what you'll need when you need to know." Then her eyes begin to glow, but not just her eyes — the Dust in her sleeves becomes a little bit brighter, but the gold of her earring becomes especially brighter, with the black crystal (glass?) gaining a whitish sheen. Someone told me that they thought she was burning Torchwick with her touch… but he doesn't jerk away as if from a source of harm, and the sound (which is actually part of the background music) isn't actually a sizzle, nor is there any sign of heat distortion, flame, or smoke.
* After Cinder's eyes, Dust, and earring begin to glow, she maintains eye contact with Torchwick for only a split second more before she looks down, as if at her hand on his chin, and her expression slackens as if she's concentrating on something before her eyes flicker back up to resume eye contact; Torchwick gives up the line of thought.
Volume 2, Chapter 7 "Dance Dance Infiltration"
* The Dust in Cinder's clothing flares up as she creates glass swords from fire. Her eyes do notglow.
* Cinder pulls a canister of yellow Dust (or simple sand?) from her thigh, tosses it into the air as a cloud, and then condenses it into transparent crystal/glass shards with jagged yellow veins. They shatter without noticeable effect when Ruby parries them.
* Cinder blocks Ruby's rifle shots with her palm again, and the Dust in her sleeves lights up when she does so, which suggests that it is a Dust technique.
* Cinder repeats her trick of using Dust to create glass swords from fire as her clothes light up. Then both her clothes and her eyes light up as she creates three arrows. Her eyes and clothes rapidly dim before she fires the arrows, which create a fiery explosion when they hit the ground where Ruby was standing. There is no shrieking hell-portal.
Volume 2, Chapter 8 "Field Trip"
* Ruby said, "I know she fought with glass. I don't think that was her Semblance, though."
Volume 3, Chapter 7 "The Beginning of the End"
* Cinder used a powerful glass attack against Amber. So it isn't a magical power.
Volume 3, Chapter 13 "The End of the Beginning"
* After Cinder becomes a Full Fall Maiden, the last time the Dust in her sleeves is ever used on screen is her big final (?) attack against Ozpin, when she presumably wanted to throw everything she had at him to put him down. But against Pyrrha? No Dust-glowing. Either Cinder used it all on Ozpin or, since she clearly didn't consider Pyrrha a threat, Cinder just didn't care to use it.
* Cinder tries to soothe the Dragon, or perhaps even hypnotize it. Her eyes do not glow. The Grimm squawks back at her in a softer tone than its previous shriek, as though it suddenly felt less hostile, and RoosterTeeth's official closed captioning confirms that this was their intent. Yet Cinder's expression falls into a frown, she backs away from the monster, and she summons a flame into her hand, as though she were about to fight the beast before Pyrrha's arrival distracts her. Did her attempt at controlling it fail, or is controlling the Grimm a two-step process that requires her to first soothe it into defenselessness and then defeat it?
* Pyrrha's sword in Cinder's hands starts to glow where she holds it, with her eyes starting to glow a moment afterwards. Not "Flaming Maiden Eyes", but just the simple glowing irises we've seen from the beginning. The Dust in her sleeves does not glow. As a reminder, by this point we've seen Cinder use Maiden powers to levitate and jet-fly around without revealing Flaming Maiden Eyes.
In fact, there doesn't appear to be a rhyme or reason about when when Cinder does either the Glowing Eyes thing or the Flaming Maiden Eyes thing.
When Amber went "Avatar State", her eyes kept flaming until her Aura broke, and then they started flaming again when she recovered enough to keep fighting back with her Maiden powers. After she put Emerald down, her eyes went back to normal — presumably, whatever non-Aura fuel her Maiden powers used must have run out, because Amber grabbed her staff and limped over to Emerald to cave her skull in, as opposed to hitting her with another fireball or lightning bolt.
But with Cinder, the Flaming Maiden Eyes come and go. They never appear when she's only flying or floating with fire, and they don't always appear when she's hurling fireballs or conjuring shards of glass. At one point, as I mentioned earlier, Cinder's eyes glow rather than flame when she's melting Pyrrha's sword in her hands.
What's funny is that both kinds of special eye could be stacked together. Flaming Maiden Eyes are just flames around a Maiden's eyes; their irises remain unchanged. Cinder's irises seem like they could glow noticeably even while surrounded with flames.