It must have two conductive plates separated with an insulating dielectric in order to behave like a capacitor. While some plates are more efficient than others, and some dielectrics perform better than others, these are the minimum requirements.
The minimum requirements are not met by a solid block made of silver, plastic, or ice cube. They are therefore not capacitors.
A cloud is a conductor plate that’s placed over some atmosphere, which acts as an insulator. Earth, on the other hand, is another conductor plate. Now there’s a capacitor. If you charge it enough, the dielectric will explode and you’ll get some lightning.