Steel, stainless steel A2 or stainless steel A4.

Steel grade 8, bright or galvanised.
Steel grade 10, bright or galvanised.
Steel grade 12, bright.
Stainless steel A2-70, bright.
Stainless steel A4-70, bright.

The grade of a nut must always be the same or higher than the grade of a bolt. That is, bolts in grade 8.8 are always paired with a nut in grade 8 (or higher, but never lower).
The maximum nut height H max. is shown in the table.

DIN ISO 272 spanner sizes.