On the banks of the River Thames, when the tide is low, a person walking along the shore can see all kinds of things. With a keen eye, you can spot blue-and-white shards of 19th-century pottery, delicate stems of 18th-century clay pipes, brass buttons from coats, and coins dating back to the Romans. And if you look in the right spot on a sunny day, you might see something special: the wink of tiny, dark red stones, shining like pomegranate seeds against the pebbles. If you see these, consider yourself…
