The word for "metal" was *h2ei-es-, according to Mallory & Adams, or *h2éios, via Beekes. No matter which reconstruction you prefer, the word was responsible for a number of 'metal,' 'copper,' 'iron,' and 'ore' reflexes.
Outside of the word for "metal" itself, we find that metal words are based upon the colors yellow, white, and red.
*ǵhelh3- "gold" but literally "the yellow." (Lendinara 2007)
*h2erǵ-nt-om- "silver" but literally "the white." (Mallory & Adams 1997)
*h1roudhós "copper" but literally "the red." (Mallory & Adams 1997)
Much has been written and much more remains to be said about ancient metalcraft. Few knew that the PIE metal-system was based on colors. We could suppose that perhaps other metals and materials were based on colors as well, but that requires more research for another blog post.
"metal," "silver," "copper" in Mallory, J. P. & Douglas Q. Adams. The Encyclopedia of Indo-European Culture. Fitzroy Dearborn Publishers. 1997.
Beekes, Robert S. P. Comparative Indo-European Linguistics: An Introduction. John Benjamins Publishing Co. 2nd Edition. 2011.
Lendinara, Patrizia. "The Survival of Indo-European Words in Old Frisian" in Aspects of Old Frisian Phonology. Ed. by Geart van der Meer, Rolf Bremmer Jr., & Oebele Vries. Rodopi. 2007.