The cation of the metal, which gives green colour bead in cobalt nitrate charcoal cavity test is :

Zn forms Rinmann's green $-\mathrm{ZnO} · \mathrm{CoO}$

Because heating metal carbonate produces a colourless residue that cannot be positively identified as ZnO, cobalt nitrate testing is used to determine zinc instead of the charcoal cavity test.

$\mathrm{ZnO}+\mathrm{CoO}\to \mathrm{ZnO}\cdot \mathrm{CoO}$

The compound formed is green.