It is during an intense session of earthbending practice with Toph that Aang feels it.
It distracts him thoroughly enough that Toph is able to knock him down with a well-aimed rock shower. As he is struggling to his feet, Toph puts her hands on her hips and demands petulantly, "What was that, you dummy? You lost your focus."
"Yeah, sorry," he mumbles, trying to put his mind back on the sparring session. He closes his eyes and listens to the vibrations in the earth—and feels it again, that curious sensation of click click click. The vibrations are hitting something even more solid than the earth he and Toph are bending.
His concentration is shattered again, and he opens his eyes to ask Toph what it is—but she has bended a large pebble at him and it smacks him right between the eyes. He loses his balance and falls again.
Toph blows out an exasperated breath. "Wow, that was even more horrible than the last time," she says sarcastically. "Nice job, Airhead. What is up with you tonight?"
Lying flat out on his back, Aang doesn't even try to get up this time. He closes his eyes and sends all his earthbending energy downdowndown—and hits that spot again with a resounding plink. To his utter surprise, the energy rebounds back at him, doubling the vibrations of the earth and making his head swim momentarily with disorientation.
"Toph," he says, when the dizziness has passed and he can open his eyes again, "do you feel that? Those weird vibrations?"
She sighs, but listens to the vibrations. After a moment, she says, "So?"
"What is it?" Aang sits up and blinks at her. "I've never felt anything like that before."
Toph shakes her head. "It's a gemstone deposit of some kind," she says carelessly. "Probably diamond. Why?"
"Yeah, judging from how hard the stone is and how quickly the vibrations are rebounding off the surface. Diamond is the hardest natural stone there is."
Aang frowns. "I've heard of diamonds," he says. "I think I've seen some really, really small ones before. They're the shiny white ones, right?" Toph makes an impatient noise, and belatedly he remembers that she wouldn't know the visual characteristics of the rock. "But this one feels bigger. And it feels—" he considers his word choice, "—rough."
"It's unrefined." Toph shrugs. "Uncut and rough. It's not going to look like the cut diamonds you see in jewelry. It's going to look like a regular rock."
"Jewelry," Aang echoes her. "That's right. Don't people wear diamonds and stuff?"
"Yeah," says Toph. "In rings and necklaces and all kinds of stuff. It's recently become a popular trend in the upper society circles for men to give women rings with diamonds set into them for betrothal gifts."
"Really?" Aang is interested.
"Yeah." Toph lifts a hand and lets it fall. "My father gave one to my mother when he asked her to marry him. She wears it for important society functions and stuff."
"Cool…Hey, Toph, can we bend it out?" Aang says excitedly. "I've never seen an uncut diamond."
Toph sighs loudly. "Fine, fine. But after this, we're going back to our training."
Together, they focus on the odd click click vibrations of the diamond and, with laborious effort and strain, bend it out of the ground.
The diamond fascinates Aang. It holds a hint of the luminescent beauty that it would show if cut and polished; right now it just looks a very shiny rock. It is about the size of his thumb, and it is easily the prettiest stone he has ever seen.
"Wow," he says reverently.
"Yeah," says Toph boredly. "Are you done?"
Aang pockets the diamond and resumes his training.
Later that night, when everyone else is asleep, Aang is still awake, idly toying with the diamond between his fingers. It catches the dying firelight and shimmers with promise.
He has been careful not to show anyone the stone; for some reason, he just does not want them to see it yet. It's not as if he wishes to keep it to himself. He just…
He sighs, unsure even now why he hasn't mentioned the diamond. Thankfully, Toph hasn't either. Then again, she wasn't nearly as impressed with it as Aang is.
He tilts his head to one side and regards the jagged edges of the diamond curiously. Toph, he recalls, was easily able to shape and reshape the space rock. Did that mean that he could…
Wrinkling his brow, he concentrates his energy on the stone. Nothing happens except that curious plink plink vibration as the energy bounces off the surface. He frowns, disappointed.
"Diamonds are the hardest natural stone there is."
Humming thoughtfully, he takes a deep breath and intensifies his energy on the diamond's surface—and to his utter surprise, the top of it flattens and smooths until it resembles a pane of glass.
"Whoa," he says, taken aback. Then he grins, wide and delighted. Holding it cupped between his hands, he shapes and reshapes the diamond until it looks something like the tiny ones he's seen before, set into rings and small brooches. It is teardrop-shaped, a glittering glassy white, and slightly cloudy. It is still the prettiest stone he's ever seen, and he thinks it even more beautiful for its impurity. With a laugh, he holds it up to catch the last of the firelight.
"Wow, Aang, that's really pretty."
He nearly drops it into the embers. He whirls around to see Katara standing behind him, rubbing one eye and smiling sleepily at him.
"You startled me," he says, surprised. He had not sensed or heard her approach, or even her awakening.
She shakes her head. "You were pretty focused," she says offhandedly. She takes a seat beside him and peers at the stone.
"Toph and I found it today during practice," he tells her quickly, blushing for some unknown and unfathomable reason. "We bended it out and I kept it and it used to look like a regular rock, but—"
"I saw," Katara interrupts, smiling. "You're getting pretty good at earthbending. Isn't diamond supposed to be the hardest mineral there is?"
"Yeah," he says proudly, "and it took me a little while, but I figured out how to bend it." He hesitates, then philosophically hands the diamond to Katara.
She examines it, turning it over and over in her hands, watching the play of golden light over its surfaces. "It's really pretty, Aang," she says again, and extends it toward him.
"Keep it," he says before he can process the thought fully.
Katara's eyes widen. "Oh, no, Aang, I couldn't," she protests. "You worked really hard on that, I watched you—"
"No, really," he insists, smiling at her as the idea of it—of giving Katara this diamond as a gift—grows on him and begins to look better by the second. "I want you to have it."
"Are you sure?" she says after a moment in which she obviously debates ways to refuse and just as obviously fights her own delight.
"Yeah." He beams at her. "What I am supposed to do with it?"
She laughs. "What am I supposed to do with it?"
He blinks, then flushes slightly. "Well. Toph says that…" He stops, appalled that he'd been about to relate the story of Toph's father and mother. "Uh. You can put it in jewelry. Or something."
"She's right," Katara says absently, running a finger over the diamond's edges. "But this one is bigger than any diamond I've ever seen. I don't think it can be set into a ring or necklace without looking gaudy."
"Oh." Aang is acutely disappointed. "Well. You—you can keep it anyway."
She laughs. "I will." She leans over and kisses his cheek. "This is very kind of you, Aang."
"Uh. Right," he says, willing himself not to blush any harder than he is already.
"I don't know what I'm going to do," she teases. "You've given me a beautiful necklace and now a diamond. How will I ever repay you?"
He looks away. "I hope you don't think of it like that," he says quietly. "They're gifts, Katara, not debts."
She puts a hand on his shoulder, alarmed at his tone. "I know that, Aang," she says. "I was just kidding."
He smiles at her. "Okay."
She looks relieved, and stands up. "I'm going to go back to sleep. You should get to bed soon too."
"I will." He watches her pad back to her sleeping bag and turns back to the fire, which is nothing more than glowing coals by now.
He thinks that this is the second time he's given her what is essentially an engagement present.
Maybe the next time he does, they'll both be ready for it.