What type of matrix is typically found as a layer in finished boulder opal?

Boulder opal is predominantly associated with ironstone, which serves as the matrix in which the opal is found. Ironstone is a sedimentary rock that is rich in iron and often features a banded, layered appearance. In the case of boulder opal, the opal forms within the natural cavities and fractures of ironstone, creating beautiful inclusions and patterns. This unique association contributes to the overall beauty and value of boulder opal, as the contrasting colors of the opal against the ironstone matrix enhance its visual appeal.

The other types of rocks mentioned in the choices, such as limestone and sandstone, do not typically provide the same conditions necessary for the formation of boulder opal. While granite is an igneous rock, it is not a suitable host for opal. Therefore, ironstone is the correct answer as the primary matrix found in finished boulder opal.