Beyond the slumpstone wall lay a backyard, a swimming pool. Dappled with morning light and tree shadows, the water glimmered in shades of blue from sapphire to turquoise, as might a trove of jewels left by long-dead pirates who had sailed a sea since vanished.
This is a swimming pool. A. Swimming. Pool. Odd happens to run past it while persuing someone. I would expect this description for some sort of mythical lake that plays a big part in the story (although it's still not a very good simile).
