To accurately identify "C. diphtheriae", a Gram stain is performed to show Gram-positive, highly pleomorphic organisms with no particular arrangement. Special stains like Albert's stain and Ponder's stain are used to demonstrate the metachromatic granules formed in the polar regions. The granules are called polar granules, Babes Ernst granules, volutin granules etc. An enrichment medium, such as Löffler's medium, is used to preferentially grow "C. diphtheriae". After that, a differential plate known as tellurite agar, allows all "Corynebacteria" (including "C. diphtheriae") to reduce tellurite to metallic tellurium. The tellurite reduction is colorimetrically indicated by brown colonies for most "Cornyebacterium" species or by a black halo around the "C. diphtheriae" colonies.