The specifications of bullion is regulated by market bodies or legislation. In the European Union, the minimum purity for gold bullion which is treated as investment gold with regards to taxation, is 99,5% for gold bars and 90% for gold coins.
The price of gold bullion is influenced, by demand from companies who use gold to make jewelry, and other products, and by perceptions of the overall economy. For example, political, or financial instability, affect the price of gold bullion.