The odd one out is Coral.
Coral is a living organism, while diamond, ruby, and emerald are all minerals.
Coral is made up of tiny organisms called polyps. Polyps secrete a hard, calcium carbonate skeleton that forms the coral reef.
Diamonds are made of carbon that has been subjected to intense heat and pressure. They are the hardest natural substance known to man.
Rubies are made of corundum, which is a type of aluminum oxide. They are red in color because of the presence of chromium.
Emeralds are made of beryl, which is a type of beryllium aluminum cyclosilicate. They are green in color because of the presence of chromium and vanadium. | 
