The contrasting optical properties of diamond (transparent due to a wide band gap) and graphite (opaque/semimetallic due to a zero or very small band gap) are well-established physical facts; no citation is needed.
The claim states basic solid-state physics regarding carbon allotropes (diamond is a wide-band gap insulator/semiconductor and thus transparent to visible light, whereas graphite has a zero band gap / semimetallic π-electron network that absorbs visible light). This is a textbook piece of common scientific knowledge and everyday observation, making citation unnecessary according to Step 0.