Answer
See work below for details.
Work Step by Step
Heating wood in the absence of air produces charcoal. Charcoal contains a high amount of amorphous free carbon and retains the general overall shape of the original wood, but the process creates many voids, resulting in a much lower density. The voids within charcoal create high surface area that makes the charcoal useful for filtration. The impurities in a liquid or gas adsorb on the charcoal surface as the liquid or gas flows through the pores in the charcoal. But diamond is an extremely strong network covalent solid, where all of the carbon atoms are covalently bonded. This does not allow diamond to act as a good filter.
