Laser emission from Ruby crystal is because of transition:

1. from conduction band to valence band of Al₂O₃
2. from conduction band to one of the levels due to Cr ions
3. between energy levels introduced by Cr ions
4. between one of the levels due to Cr ion and valence band of Al₂O₃

Laser emission from Ruby crystal is because of transition between energy levels introduced by Cr ions.