This Boulder Opal exhibit yellow and blue colors as a result of its distinct play of color. It also exhibit the 'opalescence', which is typically a milky, whitish blue color veins. This opal pendant resembles a painting picture of a sunset sun on blue waters of our Mother Nature. The back of the pendant is open i.e you can see and touch the Boulder Opal. Boulder Opal is theNatural BOULDER OPAL and Citrine Crystal, 925 Sterling Silver Pendant Jewellery. official birthstonesfor the month of October, along with all of the other varieties of opal gems.