From halam (in the sense of hardness); a precious stone, probably onyx -- diamond.
see HEBREW halam
H3095. yahalom
יַהֲלֹם noun [masculine] a precious stone, jasper ? (so ᵑ9 ᵐ5), or onyx? (compare Di Exodus 28:18) (apparently from v הלם; owing to its hardness, as able to smite, hammer?) — mentioned in lists of precious stones 28:18; 39:11; Ezekiel 28:13.