A conductor needs free ions (electrons) to conduct.

Iron...being a metal is a conductor......coz it contains free ions ..i.e. Electrons.... 

Wood is an insulator...coz no free ions to conduct....

Wood is a poor conductor of heat (as well as other forms of energy) because it is covalently bound as a compound. As a result, it does not have the free electrons that scatter about to conduct different forms of energy like metals and other strong conductors do.

It is a bad conductor of heat when compared with iron, steal, silver etc. It is good conductor when compared with wood. For a heat conduction or electrical conduction there must be free electrons in the material. So in the case of plastics there is no free electrons, so plastics are bad conductors of heat.

I would think it depends what minerals the stone is made from. If it is high in salts and metal ore (ferric oxides & other metal oxides), then it would be a conductor because these substances exist with free electrons throughout their structure which allow electricity to flow through them. If it is mainly crystalline minerals with strongly bonded lattice structures, it should insulate well. I would imagine that if the stone had a high proportion of water in the minerals, this would decrease it's resistance. Overall, I think it is incorrect to say that stone in general is either one or the other - it must depend on what type of stone it is!

Sangeetha Pulapaka

The reason why  metals are good conductors has to do with the nature of their electrons. The outer, or valence, electrons in metals are shared by all the atoms. We call these electrons “delocalised” as they are not associated with a single atom or bond.

Below their melting points, metals have an ordered, crystalline structure. Electrons reside in energy levels, but they only reside in certain specific ones, with particular energies. In a metal, because all the atoms are the same, the valence electrons are all at the same level, so it can be considered an “energy band”. The electrons in this band, the valence band, are free to move around.

There is also a conduction band that is a higher energy level and, in metals, the two bands overlap (in terms of energy levels, not physical overlap), such that it is easy to bump an electron up a level. If energy such as heat or electricity is applied to the metal, electrons are elevated from the valence band to the conduction band, and move through the metal leading to an electric current or heat conduction.

In non-metals, the valence band and conduction band have a large gap between them, so it takes far more energy to get an electron into the conduction band. Thus, they are good electrical insulators. Metalloids, or semi-metals, are somewhere in between, leading to their ability to conduct heat and electricity but only when a good deal of energy is added.

Delocalised valence electrons can transfer heat energy as well as electric current, making metals excellent heat conductors. The metal ions are packed tightly in the crystalline structure. As the atoms vibrate more with kinetic energy (heat), they are able to pass that energy through the material more easily. Wood and plastic are non-metals. They do not have free electrons.