Opal develops within the thin veins and fissures of ironstone boulder. When the opal is too thin to be removed from its ironstone host (matrix) to be fashioned as a cabochon opal, the opal and matrix is polished intact, hence its name. Boulder opal is also referred to as opal-in-matrix. The matrix enhances durability and color vibrancy. Boulder opal can resemble opal doublets (assembled stones) but is entirely natural. Highly desirable, boulder opal is quite rare and finer specimens can display all colors of the rainbow. Boulder opal is found only in Australia.