Diamonds are mostly composed of what element?

Diamonds are composed almost entirely of the element carbon, often with some other impurities in them, such as nitrogen (i.e., pure diamonds are entirely carbon). The element carbon comes in three different natural forms, or allotropes: diamond, graphite, and amorphous.

Explanation:

oxygen, selenium, tellurium, and polonium.

they're all from the same group as sulfur (group 16).