What is it?
GAC - Granular Activated Carbon. Particles of activated carbon larger than powder.
GAC is manufactured from carbon, typically coal. it is heated in the presence of steam at very high heat. This process causes the carbon to develop huge numbers of tiny pores, which trap non-polar wastes at the molecular levels by means of adsorption and ion exchange, and removes heavy metals and organic molecules, which are the source of undesirable colours and odours, through a process known as molecular sieving.
The best GAC for filtering water is made from coal and is macro-porous (having larger pores). A good macro-porous activated carbon feels light (not dense) and fizzes and floats when initially wetted. GAC intended for removing wastes from air (such as odours) are commonly made from coconut shell and are micro-porous. Carbons for filtering air feel more dense.