Amonum nitrate and aluminum dust.
Yup fertilizer and super fine aluminum dust. You may own either but own them together you win the California felony award.
I hate this state, can't have any fun here
Penal Code 18720
Every person who possesses any substance, material, or any combination of substances or materials, with the intent to make any destructive device or any explosive without first obtaining a valid permit to make that destructive device or explosive, is guilty of a felony, and is punishable by imprisonment pursuant to subdivision (h) of Section 1170 for two, three, or four years.