October’s birthstone is prized for its unique “play of color”—iridescent flashes that sweep across the stone as the angle of viewing changes. Varieties include white opal, Mexican fire opal, boulder opal, and, perhaps the most desirable of all, black opal, whose background of black, dark blue, dark green, or dark gray makes the spectral colors on top stand out in sharp relief. In his comedy Twelfth Night, Shakespeare dubbed opal “queen of gems,” and Queen Victoria liked it so much she started an opal trend at her court. Not coincidentally, legend had it that opal was associated with wealth.