The term 'hard white metallic element' refers to beryllium, a chemical element with the symbol Be and atomic number 4. Beryllium is a relatively rare element in the Earth's crust, but it is found in certain minerals such as bertrandite, beryl, and chrysoberyl. It is a hard, brittle metal that is used in alloys, nuclear reactors, and X-ray machines. Beryllium is also used in the aerospace industry for its light weight and strength.