According to BullionVault, gold bullion is gold that is measured purely by its precious metal composition. Bullion also comes in silver, and gold bullion is normally found in bar or coin form.
GoldCore mentions that most gold bullion has a 99.9 percent rarity value, but other bullion has a rarer purity rate of 99.999 percent. The value of a bullion coin depends on the weight of the gold.
About.com notes that not all gold coins are not actual bullion. For instance, a U.S. Gold Eagle is not a pure bullion coin, since it contains other alloys such as silver and copper. A U.S. Gold American Buffalo is a truer bullion because of its 99.9 percent purity. Generally, bullion coins in the United States are intended for investment rather than circulation.