A refractory substance is one that vaporizes (that is, boils) at high temperatures or condenses from a gas at high temperatures. Refractory substances (often called simply "refractories") are used for thermal insulation and to line furnaces. Often made into firebricks, refractories are composed of various substances (mostly oxides), including the acidic fire clay (see clay), silica, and zircon; the basic chromite, dolomite, and magnesite, and the neutral carborundum, corundum, and graphite. See also ceramics.

Refractory elements include are calcium (Ca), aluminum (Al), uranium (U), titanium (Ti), the rare earth elements such as cerium (Ce), europium (Eu), and gadolinium (Gd); tungsten (W), zirconium (Zr), and thorium (Th). The opposite of refractory is volatile.