Metals are a group of chemical elements defined by the ability to form metallic bonds between metal atoms. Metals are typically lustrous, malleable, hard and conduct heat and electricity. Metals can be combined with other elements or other metals to make different substances called alloys.
Metal ores were commonly extracted in mining operations as they were useful materials in construction and ornamentation. Metals were amongst the substances in the diet of the Horta and were easily dissolved by their digestive acids. (DS9 novel: Devil in the Sky)
In 2369 the unique metals in Worf's baldric and Ro Laren's earring were useful in locating them when they was involved in a shuttlecraft accident, as the distinct signatures of the metals allowed them to be more detectable in sensor scans. (TNG comic: "Light of the Day")