- Isomorphism refers to the phenomenon where two or more substances crystallize in the same crystal structure. - Crystalline silicon has a diamond cubic crystal structure, where each silicon atom is covalently bonded to four other silicon atoms in a tetrahedral arrangement. - Diamond, which is also a crystalline allotrope of carbon, also has a diamond cubic structure, with carbon atoms replacing silicon atoms in the same arrangement. - Coke, Graphite, and Coal are other forms of carbon, but they do not share the same crystal structure as crystalline silicon. Graphite has a layered hexagonal structure, while Coke and Coal are largely amorphous or contain various carbonaceous materials without a single, defined crystal structure.