Black opal is one of the rarest gems on Earth, only found in specific locations. Australia almost reaches 100% of global production. Even with this beauty, the main mineral for opals is silicon dioxide normally called silica. It's not a secret that silicon is one of the most abundant elements. Something creates this kind of colorful opals with unique reflections.
That has relationship with crystallization, as frequently different sources inform us, due to spheres of silica arranged in a particular pattern to create light effects that lead to the display of the color spectrum, sometimes complete.
The singular characteristic of black opals appears in the presence of oxides from other elements like iron or carbon. With a black background, the light effects of refraction and diffraction responsible of the colors are better perceived by the eye.
Black opal continues being one of the preferred gems for jewelry, although it's not particularly economic. Considerable efforts are being made to create synthetic forms of opals trying to keep the beauty and avoiding the exclusivity, high price and resource exhaustion. With high technology the differentiation between natural and synthetic would be very difficult!