Iron powder, iron plate, and natural red ochre were kept at 30°C and relative humidity 100% for one year, and they were observed by scanning electron microscope. A structure consisting of hollow cylindrical shapes was found which resembled crystals found in red ochre used in paintings in the stone chambers of ancient tombs. The authors conclude that these hollow cylindrical crystals were formed in conditions of high humidity over a long period of time. Vermilion found in the ancient tombs does not show crystal shapes different from the modern pigment. -- AATA