The boulder opal is an opal that has been cut from its ferruginous bedrock. It is also called "matrix" opal.
The opal owes its iridescent shimmer to a phenomenon of diffraction of white light that breaks down into multiple colors of the rainbow. They are composed of tiny grains so that when the stone is oriented at different angles, colored bursts of light appear.
The lights that animate the opal are its value and not its dimensions. The more they are present and the less the ferruginous rock visible on the surface, the more valuable the stone.
This boulder opal has many bursts of colors.
The blur effect, is not due to a lack of sharpness of the photo, but is a characteristic of the stone, due among other layers to different thicknesses.