Slate:

The natural beauty of slate provides a floor covering to behold - though of course like all natural stone surfaces it is cool to the touch.  Therefore the integration of a uheat underfloor heating system with a slate floor is a perfect match, and the natural cool feel of this highly durable floor surface can be eliminated.  Slate flooring has excellent heat conducting properties, and combined with a good insulation board beneath (see here for details on insulation boards), the floor will look and feel wonderful.