Cummingtonite is a metamorphic amphibole with the chemical composition (Mg,Fe2+)2(Mg,Fe2+)5Si8O22(OH)2, magnesium iron silicate hydroxide.

Amphibole is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain SiO4 tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is Amp. Amphiboles can be green, black, colorless, white, yellow, blue, or brown. The International Mineralogical Association currently classifies amphiboles as a mineral supergroup, within which are two groups and several subgroups.