Molejón: The stone that sharpened knives and preserved fossils from 11 million years ago

Molejón, a stone once used to sharpen knives, also preserves a striking trace of the distant past: the fossil impression of a blade from 11 million years ago.

The stone comes from the Coris Formation in Cartago, whose sandstones began as coastal sands during the Miocene. Over millions of years, those sands hardened into rock.

Its tiny quartz grains formed an abrasive surface. Sliding a steel blade across the stone wore down the metal and restored the edge, explaining its practical use as a sharpening stone.

A photograph of the fossil shows that the blade impression appears on both sides of a block. The block split precisely along the place where the blade had been buried, preserving the trace inside the stone.