Both coal and diamonds are forms of elemental carbon. coal is brittle while diamonds are considered to be the hardest of all substances known. what is the appropriate justification for this? both coal and diamonds are allotropes of carbon, having different structures due to different arrangements of carbon atoms. both coal and diamonds are isomers of each other, having a similar molecular formula but different structures. coal is ionically bonded together while diamonds are covalently bonded together. nextreset