Hardfacing materials include particles of polycrystalline diamond (PCD) material embedded within a matrix material. The PCD particles comprise a plurality of inter-bonded diamond grains. Material compositions and structures used to apply a hardfacing material to an earth-boring tool (e.g., welding rods) include PCD particles. Earth-boring tools include a hardfacing material comprising PCD particles embedded within a matrix material on at least a portion of a surface of a body of the tools. Methods of forming a hardfacing material include subjecting diamond grains to elevated temperatures and pressures to diamond-to-diamond bonds between the diamond grains and form a PCD material. The PCD material is broken down to form PCD particles that include a plurality of inter-bonded diamond grains. Methods of hardfacing tools include bonding PCD particles to surfaces of the tools using a metal matrix material.