It is so named because it was unearthed in 1986, a year when Halley's Comet could be seen from Earth.
It was found at Lightning Ridge, New South Wales, Australia, by the Lunatic Hill Mining Syndicate.
It weighs 1,982.5 carats (396.5 g; 13.99 oz) and is about the size of a man's fist.
[1] The Halley's Comet Opal is a very fine specimen, with few flaws or blemishes and a large green and orange 1.6 cm (0.63 in) thick color bar which goes through the opal.
Formed about 20 million years ago, it is an example of a nobby, which is a natural lump-shaped opal found only at Lightning Ridge.