Caldo de Piedra is deceptively humble. Located a few miles outside Oaxaca City, the small, thatched-roof restaurant is named after its specialty, stone soup. Before it’s cooked, even the soup itself might not look like much. But when the chef plucks the glowing rock from the fire and drops into a broth swimming with ingredients, the bubbling medley brings an ancient culinary art to life. Caldo de piedra is a staple among northern Oaxaca’s indigenous Chinantec community, especially in the small town of San Felipe Usila. After catching fish and shrimp from…