A ritual mask is most often carved wood, though gourd, woven raffia backing, metal and feather ornament all appear across different regional traditions, and in the traditions where masks are most tightly restricted, carving is done specifically by members of the society that owns the mask, trained through apprenticeship rather than open to any woodcarver.
Restriction as the defining fact, not an aside
In several West African masking traditions, associated with initiation societies such as Poro and Sande, both the carving and the wearing of specific masks are restricted to initiated members, and in many of these traditions uninitiated people, women in particular in some societies, are traditionally forbidden even to view certain masks directly. Pacific Northwest Coast transformation masks carry their own, differently structured restrictions tied to clan and hereditary right to specific crests and stories, and Venetian carnival masks sit at the opposite end of the same category entirely, a secular tradition with no restriction on maker or wearer at all, worth naming here so the contrast is clear.
Why a mask is handled differently from most objects on this list
In several of these traditions a mask while worn is understood not merely to represent a spirit but to be inhabited by it for the duration of the rite, which is the underlying reason for such strict control over carving and wearing. Retired masks in restricted traditions are not displayed or discarded casually, they are kept in a restricted structure belonging to the society or ritually retired, and their presence instead in outside museum collections remains a live and unresolved point of contention for the communities they were taken from.