The athame is usually a dark or black handled knife with a double edged blade but any knife that has a personal significance can be used as long as it feels right. The blade of the athame need not be sharp as it's not used to cut anything on the physical plane. The handle and blade can be engraved with words, symbols and runes but it's not necessary to do so.
The athame is phallic in nature and so linked with the God it is also a tool of the east and linked with air it also stands for intellect, right thinking and calculation.