“You’re right. I ought to come back just to annoy you,” Tefwe said, squatting. She dipped her hand into the shaded pool where the rod of the raising screw slanted into the tawny grains. She let the sand fall back between her fingers; it slipped away almost as quickly as water would have. “It moves very smoothly,” she said, inspecting her hand. A few tiny grains adhered to her skin, all in the lines of her palm.

“Please don’t do that,” the drone said, using invisible maniple fields to adjust parts of the diamond-sheet-covered solar panels.

“Why?” Tefwe asked.

“Moisture,” Hassipura said. “And impurities such as salts. Your hands will have added a little of each to the sands.”

“Sorry.” Tefwe squatted and stuck her head down into the shade created by the solar panel, gazing at the pool of sand underneath. Inside its transparent sleeve, the turning screw seemed barely to disturb the surface of the sand, which appeared to flow in to fill even the slightest of hollows. She glanced up to see if the drone was looking, reckoned it couldn’t see, then stuck a finger into the surface of the sand pool and took it smartly out again. The sand closed up round where her finger had been – running in, again, like water – to leave no sign that its surface had been disturbed.

“Will you stop doing that?” the drone said, tiredly.

“Apologies,” Tefwe said. “How does it move so smoothly?”

“The grains are spheres,” the drone said, clicking something back into place on the solar array. “They are polished, individually where necessary. I call the stuff sand because it starts out as ordinary sand and still has the same chemical composition as the raw material, but really the particle size is reduced almost to that of fines, and the polishing process leaves each grain almost perfectly spherical. See.” The drone shifted in the air, humming very faintly.

Tefwe stood and straightened as a bright screen suddenly filled the air in front of her, seeming to dim a significant part of the sky and putting her in shadow. The drone had produced a holo display like a magically produced cabinet hovering in front of the woman. The holo showed two grains, highly magnified. One appeared to be about the size of Tefwe’s head, and was jagged, crystalline, all straight edges, spires and juts; not unlike the rocky outcrop itself. It was rainbowed with diffraction colours. The other was pebble-sized, a glass-like shiny blonde and seemingly a perfect sphere.