Objects consecrated for ritual use are believed to contain a spiritual essence or power called "nanm". The "asson" or "ason" is a sacred rattle used in summoning the lwa. It consists of an empty, dried gourd covered in beads and snake vertebra. Prior to being used in ritual it requires consecration. It is a symbol of the priesthood; assuming the duties of a manbo or oungan is referred to as "taking the "asson"." Another type of sacred object are the "thunder stones", often prehistoric axe-heads, which are associated with specific lwa and kept in oil to preserve their power.