The mystery garnets

When the tide recedes in a certain part of Central London, it exposes hundreds of red garnets clustered on the surface of the exposed riverbed.

No one knows exactly where the garnets came from or how they ended up at the bottom of the River Thames. Perhaps it was a shipment from the East Indies that fell into the river during unloading? As long as you have a permit, you may remove all the garnets you can find. They are going to be swept into the sea eventually, anyway.

You can also keep any single coin you turn up. But any other object with more than 10 percent gold or silver is treasure and becomes government property until the museums decide what to pay you for it.