Ivory is a luxury resource, but it may also be thought of as a "hybrid resource" due to its characteristics. It has some strategic resource-like functions by doubling the production speed of the Statue of Zeus and allowing the highly-powerful War Elephant, but also luxury-like function with the happiness it provides and the potential further boost with a Market.
War Elephants are more powerful than most units for their time, but require rare Ivory. Unlike Copper and Iron, however, you can see Ivory right from the start of the game, and it can very much be worth getting some just for those War Elephants.
Elephants are the largest living land mammals. At birth it is common for an elephant calf to weigh 225 pounds! In the wild, elephants exhibit complex social behavior and strong family bonds. Social hierarchy in calf cow groups is based on size and age, with the largest and oldest at the top and the smallest and youngest last. Adolescent males determine their own ranking order through jousting contests using head and tusks. Elephants communicate with very low and long ranging subsonic tones.
Elephant taming began in the Indus Valley around 4000 years ago. The first military application of elephants dates from around 1100 BC and is mentioned in several Sanskrit hymns. From the east, war elephants migrated to the Persian Empire where they stood against Alexander the Great at the battle of Gaugamela. Though they did not play a significant role in the battle, Alexander saw their usefulness and began incorporating them into his army. Later the use spread throughout Europe and elephants were used in such famous campaigns as the Macedonian wars and by Hannibal during the Second Punic War.