Is Dio heavy metal?

Could you please clarify whether the band Dio can be classified as heavy metal? Some argue that their music has elements of hard rock and progressive rock, while others insist that their sound falls firmly within the heavy metal genre. As a knowledgeable practitioner in the field of music, I'm curious to hear your take on this. Do you believe that Dio's music is primarily heavy metal, or do you think it transcends traditional genre boundaries?