What is the X in X-mas?
a cobalt sulfoarsenide mineral in which iron commonly replaces part of the cobalt [(Co,Fe)AsS], that occurs in high-temperature deposits. Notable occurrences are at Daskasan, in the lesser Caucasus, Azerbaijan; Tunaberg, Swed.; and Rajasthan, India. Cobaltite, like its relatives gersdorffite and ullmannite, crystallizes in the isometric system and is closely related to pyrite. They differ from the pyrite group minerals only because the S2 groups are replaced by AsS or SbS and are distinguishable because they are not as hard. For detailed physical properties, see sulfide mineral (table)