Ullmannite crystals are usually less than 2 mm, however larger have been identified in especially antimony rich environments.

Specimens of ullmannite were found at Sarrabus, Sardinia, Italy in 1887.

Antimony freely substitutes also leading to ullmannite, NiSbS.

Finally, the very rare mineral ullmannite, a sulphide of nickel and antimony, has been found as very bright metallic grains, but only a few specimens are known.

It occurs in hydrothermal copper-nickel-iron sulfide bearing veins associated with chalcopyrite, pyrrhotite, galena, sphalerite, pyrite, millerite, gersdorffite and ullmannite.

Associated minerals include nickeline, nickel-skutterudite, cobaltite, ullmannite, maucherite, löllingite, platinum-group minerals, millerite, pyrite, marcasite, and chalcopyrite.

Carrollite occurs in hydrothermal vein deposits associated with tetrahedrite, chalcopyrite, bornite, digenite, djurleite, chalcocite, pyrrhotite, pyrite, sphalerite, millerite, gersdorffite, ullmannite, cobaltoan calcite, and with linnaeite group members linnaeite, siegenite and polydymite.