Set in Stone, Chapter 22.
The first thing he was conscious of was his hands. The second was that they moved when he willed them to.
Stone did not move.
He looked at his palms, turned them over, flexed his fingers. The action was smooth. Unimpeded. He did not feel as though he had been elsewhere, precisely. There had been no sense of time passing, no dreamlike interval, but the last thing he remembered with clarity was the top of the tower stairs and Elizabeth closer than he had expected and her face.
He remembered her face.
He was standing on a tower parapet. It was night. There was music from the village below, faint and cheerful. Someone was talking: Lydia Bennet, some distance away, recounting something with the energetic inaccuracy characteristic of her narration. He did not attend to the words.
Elizabeth was three feet from him, watching him with an expression he could not immediately parse.
His mouth tasted of something warm and metallic.
He touched his lips. His fingers came away smeared faintly red in the dim light. Not his blood.
The inferential chain assembled itself with the methodical efficiency of long training: petrification upon direct exposure to the transformed state; intact stone body; reversal of petrification requiring the application of blood from the afflicted individual; the blood had been applied to his mouth; Elizabeth’s lower lip bore a small cut that had not been there when he had last seen her in the study.
Conclusion: she had depetrified him.
The medical brain, satisfied with this arrangement of the facts, withdrew. The personal brain arrived more slowly and found the conclusion materially insufficient.
She had kissed him while he was stone and brought him back.
He did not move. He suspected movement would require decisions he was not presently equipped to make.
“Wickham told you I was taken?” Lydia was saying from somewhere to his left, apparently addressing Elizabeth. “Which I wasn’t, obviously, I was at the festival having a perfectly nice time until I saw the lights up here and thought I ought to investigate, and honestly I think it’s rather rude of him to have used me as bait or whatever it was without asking first, though I suppose if he’d asked I might have said no, so perhaps I see his point—Mr. Darcy, are you quite well? You look as though you’ve seen something dreadful.”
“I am entirely well,” Darcy said. His voice emerged more evenly than he had expected. “Thank you.”
Lydia looked unconvinced but was distracted almost immediately by something glinting at the base of the tower. “Is that—oh, is that Wickham? In pieces? How did he get in pieces?”
“He fell,” Elizabeth said.
“From up here? But that’s terribly careless of him.”
“Yes.”
Lydia leaned over the parapet with the unselfconscious interest of someone examining a mildly surprising natural phenomenon. “I don’t think you can put him back together. Some of those bits are really quite small.”
“No,” Elizabeth said. “I don’t think you can.”
Darcy realized he was still looking at her. He made himself look at Lydia instead, which was easier and considerably less illuminating.
“Well,” Lydia said after a moment’s contemplation of the scene below, “I suppose that’s one way to end an evening. Shall we go back? I’m freezing and I left my shawl at the festival and Mary will be dreadfully smug if I catch cold.”
She started down the stairs without waiting for an answer, still talking: something about sausages, and a man with an accordion, and whether anyone had thought what Papa would say about this, because Papa would certainly have Opinions.
The sound of her voice receded.
Darcy and Elizabeth regarded one another across three feet of parapet in silence.
He knew he ought to say something. Several things, probably, beginning with an accounting of his own physical state—no residual stiffness, no loss of sensation, motor control entirely intact—and moving on to the more urgent matter of how the reversal had been effected when his research had consistently indicated that the process required not merely the application of blood but a precise admixture with other catalytic agents, none of which had been available on a tower parapet at the village festival.
He said none of this.
What he said was, “You reversed the petrification.”
“Yes.” She was still watching him with that expression he could not parse. “I’m sorry I didn’t tell you I was coming here. I thought—”
“That Lydia was taken.”
“Yes.”
“She wasn’t.”
“No.”
He nodded slowly. The self-reproach would come later, he suspected. Just now she looked tired and self-contained and as though she had already worked through the entire accounting herself and arrived at provisional conclusions he would not presume to interrogate.
“The cut,” he said, which was not what he had meant to say.
She touched her lower lip briefly. “The harpe. He didn’t mean to, I think. He was—” She paused, frowned faintly. “Flailing is probably the most accurate word.”
“Before or after you pushed him off the parapet.”
“During, I think.”
“I see.”
He did not, in fact, see. The precise sequencing of events was unclear but he found he did not require the details immediately. They would emerge in due course. Just now the salient facts were that Wickham was dead, Elizabeth was not, and Darcy had been petrified and subsequently restored by a process that should not have worked outside a laboratory with proper equipment and which had nevertheless worked in a manner he would need to examine more closely when he was not standing on a tower parapet tasting blood and trying very hard not to think about the fact that his patient had kissed him.
His soon to be former patient. The depetrification proved the catalytic principle. If the blood could reverse petrification it could, correctly applied, cure the condition. He had the missing variable. They could proceed.
He was aware this constituted a significant medical breakthrough. He was also aware that the breakthrough had required Elizabeth to throw a man off a tower, cut her lip, and kiss a stone physician in front of her youngest sister, and that his predominant emotional response to this series of developments was not scientific satisfaction.
He decided not to think about this too much.
“We should go down,” he said after a moment. “Lydia will have alarmed the household by now and your father will want to know you’re safe.”
“Yes,” Elizabeth said. She did not move.
Neither did he.
The music from the festival continued below, faint and incongruously cheerful. A waltz, possibly. He had never been particularly good at identifying music by ear.
“Darcy,” Elizabeth said.
“Yes.”
“When you came up the stairs. You saw—”
“Yes.”
She nodded once, a small precise motion. “Were you afraid?”
“I do not recall that I had time to be.”
Something shifted in her expression: relief, possibly, or recognition, or something he did not have a name for and suspected he would be thinking about at length later when he was alone and no longer tasting her blood.
“That seems fair,” she said.
They went down.

Is Lydia unaware that Mary was petrified a day ago?
Good continuity catch! I handwave it as Lydia mostly being off in her own little world and not paying much attention to the details of what’s going on around her. Also Mr. Bennet and Dr. Darcy have been trying not to alarm the others, so to the extent the younger sisters are aware of what’s going on, it’s probably been described as weird, but fixable.