any of a group of rock-forming minerals, as hornblende or actinolite, composed largely of silica, calcium, iron, and magnesium: they are common constituents of igneous and metamorphic rocks
See amphibole in American Heritage Dictionary 4
(ămˈfə-bōlˌ)
noun
Any of a large group of structurally similar hydrated double silicate minerals, such as hornblende, containing various combinations of sodium, calcium, magnesium, iron, and aluminum.