Final answer:
Bananas are a good source of the metal ion potassium, which is a reactive, dark blue solid in elemental form.
Step-by-step explanation:
Bananas are a good source of potassium, which is a metal ion. In elemental form, this metal ion is a reactive, dark blue solid. The transition metal ions of manganese (Mn), iron (Fe), and copper (Cu) are known to exhibit vibrant colors. These metals form complexes that are responsible for the coloration.