Opal Stone

While asterism in other gemstones, like this sapphire, is caused by inclusions, the star effect in opals is caused by the diffraction of light from imperfections in the arrangement of silica (SiO2) in the stones. "Star of India," sapphire, by Katie Munoz is licensed under CC By-SA 2.0

Article: Pricing Information For Star Opals From Spencer, Idaho

Credit: Katie Munoz