A name given to a group of magnesium-iron silicate minerals crystallising in the orthorhombic system. Crystals are usually of simple habit, a combination of the dipyramid with prisms and pinacoids. The lustre is vitreous and the colour olive green, giving rise to the name olivine. Hardness is 6½–7 on Mohs scale; specific gravity is 3.27–3.39, increasing with increase in iron content.