Black Opal has a hardness of 5-6 on the Mohs scale, and the transparency relies upon the sort and nature of the stone. Historically, Black Opal was considered for its magnificence and exciting example. These stones are in tracked down sedimentary rocks. The minuscule silica circles that cosmetics Black Opal are layered one on the other. Cause the delightful tints by diffracting and refracting light these circles.