Black opal is a rare and valuable form of opal that exhibits dramatic play of color effects when turned in the light. Opal forms when silica spheres of different sizes in a stone reflect and refract light, producing flashes of color. Black opal is particularly prized for highlighting these effects. Most black opals come from mines in Australia, though other sources include Mexico, Guatemala, and Honduras. An opal's value depends on its body color, dramatic color effects, and lack of flaws - a flawless black opal with bright, uniform colors can be worth more per carat than a diamond.