Opal stone value and quality is determined by a combination of factors including color, color play, pattern, clarity, carat weight and rarity. Strong and vibrant colors are more valued, especially black opal, which is rare and has an intense play of color. Color play refers to patterns and colors that occur when the opal is moved or illuminated, and a complex color play increases value. Unique patterns like "harlequin" are particularly sought after. In addition, clarity is an important factor for opals, as is the size of the stone measured in carats. Both factors have a significant impact on the value of an opal stone. Rare opal types such as black opal and high-quality boulder opal are more valuable due to their rarity.