Metallic lifeform
A metallic lifeform was a classification of microorganisms that had a cellular structure composed of metals. 

These lifeforms were purely speculative to the Federation until 2376, when an away team on the  found fossilized metallic microbes in a rock inside a graviton ellipse. The rock was billions of years older than the Earth and estimated to originate from the time of the formation of the Milky Way Galaxy. ()

de: Metallische Lebensform
Lifeform classifications