"Come on, Donna, it's perfectly safe," the Doctor claimed. She remained skeptical.

"That glass is less than a centimeter thick!"

"It's not glass, Donna," he protested. "It's diamond! I told you already, this is the best way to see the reefs, in the diamond submarine."

"But it looks like I'd fall straight through, into the water," Donna pouted. "Oh, all right then, spaceman. I'll trust you this once."

"Never again?"

"Shut up, you." Donna reached down for the Doctor's hand. He was standing in the half-submerged, perfectly clear submarine, and he helped her climb in. They were the only ones on the tour, and the ship was automated: no tour guide. "This is so weird," Donna whispered as she took a seat on the floor. The Doctor sat next to her, grinning happily. "I mean, it looks like I'm just sitting underwater!"

"Well, you are," the Doctor corrected. "Just inside of a diamond submarine."

She sighed. "You really like saying 'diamond submarine', don't you?"

"Who wouldn't? How often does one get to say, 'Oh, look, we're in a diamond submarine!' It's so neat. It's a submarine, made of diamond. That's some ingenious, impressive technology." The Doctor glanced up as the crystalline roof of the small underwater craft slid automatically into place. "We're going!" He smiled at her, and she couldn't help returning the grin. She took his hand and squeezed it. As she looked away from his face and out into the sea, she gasped with wonder.

The walls of the craft were so clear, it really did seem like they were sitting in the water. Gazing down, Donna noticed that the grains of sand speeding by under her were tiny fragments of gem in every color imaginable. They caught the shimmer of the pure white sun so beautifully and filled the crystal ocean with rays of rainbow. Even as they got deeper and deeper, the light didn't fade because of the complete lack of sediment in the water.

"Is there any life in this sea?" Donna asked, softly, as if in reverence for the beauty that surrounded her. The Doctor had to tear his eyes away from the sea to answer her.

"Yeah," he said. "But you see, the only carbon on this planet is in the diamonds that make up the reef we're going to, so it's silicon-based life, not like on most planets. It survives on chemical synthesis. Some are plantlike, some animal-like, but nothing sentient until the tourists came."

"Tourists?"

"Space travelers. They used a terra-former to transform the atmosphere to breathable to humans, though it's a bit thinner than you're used to. We aren't the only ones who want to explore."

"Doesn't that hurt the natives?"

The Doctor shook his head. "Naw, cos you see, the atmosphere is completely arbitrary to the composition of the liquid. It's not really pure water, it's a different compound. I'm forgetting the name, but it's not really poisonous to humans. Though don't go swimming in it, or you could get a nasty rash."

"Well, that's good to know, spaceman," Donna commented sarcastically. "So, if there are living things here, then where are they? And why aren't we seeing them?"

"Aren't you?" the Doctor said mysteriously. "Look again."

So Donna stared again out into the sea. At first, she didn't see anything. Then: "But! Doctor, there's things, clear things, just…drifting! I can't believe I didn't see them before, what are they?"

The Doctor pressed his hand against the diamond wall. It was cool, and he watched the ripples in the liquid as their craft kept speeding along, the flat ocean bottom teeming with both jewels and the transparent creatures, which weren't really perfectly clear but shone with pastel glows that were dim in comparison with the radiance of the 'sand'. Humans just didn't have the observation skills to see them until they were pointed out. But the Doctor didn't tell Donna that, or she would have slapped him. "There are more of them now. I guess they get more concentrated as we get closer to the reef." Actually, he had no idea, but as long as he pretended to know things, people didn't call him out on it. Or, that was how it often worked.

"Where is the reef?" Donna remarked. "I'd have thought we'd be there by now."

"I'm sure we're close," the Time Lord responded absentmindedly, distracted again by the dazzling colors and miraculous life outside of the capsule. He could just barely make the reef out in the distance, but then, his eyes were much better at filtering light waves than Donna's.

Then, suddenly, the submarine sped up. The force made both the Doctor and Donna slide backwards a little, and both of them laughed. "Allons-y!" the Doctor said.

"Why're we going so fast?" Donna yelped.

"No idea, but it's more fun this way!"

And then both of their breaths were taken away. They gazed in awe at the towering crystal reef that had seemingly just appeared around them. Their craft slowed down to let them take in the awe-inspiring sight of the impossible tall towers. Some looked to be made of ruby or emerald, and even a few deep sapphires, but most of pure diamond. The prisms caught the light and made the ocean light up with the most glorious colors Donna had ever seen in her life.

"Wow…" she breathed. "That's…amazing."

The Doctor's ancient eyes were alight with childish joy. "Isn't it just."

She reached over and took his hand again. He squeezed softly. They sat like that for several minutes, just taking in the wonder of the universe. "Thank you so much," Donna whispered to her best friend. "I'll never forget this moment."

Thank you for reading! This is the 6th in the series of one-shots I'm writing, one with each Doctor and companion(s) in New Who. The first five are The Red Bicycle, Faith and Trust, Nobody Was Done Dancing, Admit Defeat, and Saving One Life. I don't know when the next one will be up. I haven't written it yet, and I'm not sure what's going to happen in it. It is going to be the Tenth Doctor alone, and if anyone has any suggestions, please please leave them in reviews or PM me. I'd love to hear from you.