Black opal is rarer than the white or "milk" opal. Its blackness recalls the night sky, its bright specks suggestive of the universe beyond.

The largest uncut black opal in the world—the Halley's Comet Opal—was found in 1986, when Halley's Comet was visible from Earth.

It's something to imagine men unearthing this great stone, tiny and toiling, while this giant ball of light tore its way through the outer expanses of the universe overhead.