What is thought to cause the background color of fire opal?

The background color of fire opal is primarily attributed to iron impurities within the gemstone. When iron is present in the structure of the opal, it can impart a yellow to orange or even reddish hue, which is characteristic of fire opal. The presence of these iron ions affects the light absorption and reflection properties of the stone, leading to the vibrant colors associated with fire opals.

While other factors such as silica structure and water content can influence the overall appearance and certain characteristics of opal, it is the iron impurities that are most directly responsible for the distinctive background pigmentation. This understanding helps in distinguishing fire opal from other types of opals, which may present different colorations and effects.