CHRISTINA HARRISON SETS her spade into the ground at a sharp angle, pushes, and lifts. A load of dirt drops onto a frame strung with wire mesh. She shakes the sieve until all that remains on top are what look like a few odd-shaped pebbles. Harrison picks one out and holds it up. It takes a closer look, but then you see it: Under the caked dirt, the pebble's surface is crisscrossed by lines of tiny bumps. It's not... More >>>