Opals are formed when water and silicon dioxide ... Highlights: This is an extremely rare gem in the Taaffeite family, and until recently, only eight stones were known to exist.

Following is a transcript of the video. Narrator: High-quality black opal can cost over $10,000 per carat, making it one of the world's most expensive gemstones. But mining black opal isn't easy.

High-quality black opal can cost over $10,000 per carat, making it one of the world's most expensive gemstones. Over 90% of black opals come from Australia, but they aren't easy to find.

The two men are standing in the middle of the sun-bleached South Australian outback, hunting for opals, the rare gemstones that can sparkle with a rainbow of different colours. They sift through ...