A member of the amphiboles, Ca2(Mg,Fe)5(Si4O11)2(OH,F)2, with the ratio Fe/Fe + Mg = 0.9 to 0.5, belonging to the tremolite–ferroactinolite series of Ca-rich amphiboles; sp. gr. 3.0–3.4; hardness 5–6; monoclinic; light greenish-grey to dark green; white streak; vitreous lustre; habit acicular, often fibrous and felted; cleavage prismatic, good {110}; occurs widely in low- to medium-grade schists and some igneous rocks. The asbestiform variety is called nephrite and such felted forms were used in the past for insulation and fire-resistant materials, but the development of asbestosis in workers has severely restricted their use.