Which Is More Expensive: Opal or Moonstone? Opal is a mesmerizing gemstone known for its captivating play of color; it is an optical effect that has a wide range of colors. This phenomenon happens due to the diffraction of light as it passes through tiny silica spheres within the gemstone. Opals are found in Australia, which is a primary source of high-quality Opal. The price of these astonishing gemstones is determined by factors such as the intensity and variety of colors, the presence of any visible flaws or cracks, and the overall transparency of the stone.