Part used as spice in Mace is

Part used as a spice in Mace is aril.
Mace, a spice consisting of the dried aril, or lacy covering, of the nutmeg fruit of Myristica fragrans, a tropical evergreen tree. Mace has a slightly warm taste and a fragrance similar to that of nutmeg.
Mace is the lacy outer layer encircling a whole nutmeg seed. Also called the aril, this outer layer can be removed, dried, and used as a spice in its own right.