Lamda, the eleventh letter of the greek alphabet, is used to denote how well a material can conduct heat. The rate heat transfers though a material is measured in Watts per meter Kelvin (W/mK).  A high λ indicates the material is conductive,  a lower λ, the better the insulation. Steel has a λ=54, for wool λ=0.07, If we look at limestone it rages from 1.26-1.33.