Unveiling the Mystery of Black Opal: Facts and Legends Dark colors, often ranging from black to bluish-gray, are characteristic of black opal, a form of opal. Additionally, the stone comes in dark green or deep gray tones. The stone is well-known for its potential therapeutic benefits in addition to its exquisite look. It has been worn as jewelry by people for both decorative and spiritual reasons. In addition, the stone is used as décor. Trace levels of iron oxide and carbon are responsible for Black Opal ' distinct blackness, which sets them apart from typical opals.