The base of the Giannini stove tops can darken if the unit is left over a heating element without water in the boiler. This can occur if water is not put in the boiler or if the unit is left on the stove beyond the time of the brewing cycle completion.
These darkened marks can be minimally reduced if the unit is cleaned following these guidelines here.
Always ensure to remove the stove top from the heating element once the brewing cycle is complete.
Below are examples of boilers that have been burned due to overheating.