Metallic lifeform

From Memory Alpha, the free Star Trek reference.

A metallic lifeform is a classification of organisms that have cellular structures composed of metals.

These lifeforms were purely speculative to the Federation until 2376, when an away team on the Delta Flyer found fossilized metallic microbes in a rock inside a graviton ellipse. The rock was billions of years older than the Earth. (VOY: "One Small Step")