Daisy head colonies are seen with

A round grey or black colony with a narrow translucent scalloped border, typically produced by Corynebacterium diphtheriae on tellurite blood agar. Tellurite inhibits the growth of most other bacteria, acting as a selective agent. Diphtheria bacilli reduce tellurite to metallic tellurium, which is incorporated in the colonies giving them a grey or black colour. Reference: Textbook of Microbiology; Anathanarayan and panicker&;s; 9th edition; Page no: 237