Common heat-absorbing materials include precious metals such as silver, copper and gold. Materials that conduct more heat are capable of absorbing heat before transferring it. The most conductive naturally occurring material is diamond, followed by other pure metals. The most conductive material known is helium II, a superfluid isotope of helium that has over 45 times the absorption of natural diamonds.
The ability for a material to truly absorb heat and store it is different from its ability to conduct it. Conduction is the ability to take in heat and allow it to pass into something else. Materials that can absorb heat and then store it for a long period are called phase-change materials, which store heat when changing between solid and liquid states. Phase-change materials include silver, copper, gold, aluminium, zinc, lithium, iron, lead, titanium and water.
Heat conductivity is measured in units of watts per meter per Kelvin or (W*m^-1*K^-1). Diamond has the most at 2,200 watts per meter per Kelvin of any natural material, and is able to go up to 41,000 when enriched by certain isotopes. Pure silver gets up to 430 watts per meter per Kelvin, copper up to 400 and gold up to 318. The isotope helium II has over 100,000 watts per meter per Kelvin, making it the most conductive.