<b>Self-healing</b><b> mineral film</b><b>on</b><b> sandstone</b><b>: </b><b>a</b><b>natural passive sampler</b>

Despite experiencing thousands of years of wind and rain, some stone carvings have remained remarkably well-preserved. Understanding the information recorded is of great significance for reconstructing the level of anthropogenic pollution in the environment in the past. Here, a black to reddish-brown, relatively dense mineral film with a thickness of approximately 150 μm was observed on the surface of sandstone.