Heat transfer (or flux) = Thermal Conductivity x [Temp(you) - Temp (sleeping surface)]
Thermal conductivity is the inverse of Thermal Resistivity (degree of insulation). Ie, if the sleeping surface's temp is close to the temp at the bottom of your pad, there won't be enough temperature differential, and therefore heat transfer, for its thermal conductivity to matter much.
Two things probably matter a lot more than the surface you sleep on: a) cutting the wind to reduce convective heat transfer or b) reducing the amount you radiate to the night sky by reducing the overall area of sky you're directly exposed to line of sight - by sleeping next to or under objects that are relatively warmer than outer space, like a tent, trees, or terrain. Clouds work, too, if you can summon them up, but that option has its downsides....
Also, dark volcanic rocks are pretty good at absorbing sunlight in the visible wavelengths during the day and re-radiating that energy as infrared at night. Any rock will do this, but basalt is particularly good because of its finer crystal structure, higher iron content/density, and darker surface. This re-radiation can reduce your conductive heat loss after a sunny day if the wind or cold doesn't suck that heat out before you bed down.