Opal
The iridescent opal is similar to labradorite, but its basis is white rather than grey, though there are some darker ones.They are the birthstones of people born in October.
Many opals are mined in Australia and they are incredibly varied. The largest yet found, the Olympic Australis, was discovered in South Australia 1956, an Olympic year in Melbourne. It contains about 7,000 carats' worth of gem, weighs 3450 grams and is worth about 2.5 million AUD.
A dark opal with a suggestion about it of the night sky, the Aurora Australis was found in 1938 in New South Wales. Brilliantly coloured, it weighs 180 carats and is valued at about a million Australian dollars.
The Halley's Comet Opal is a huge and nearly flawless uncut black gem, was found around the time the comet crossed the sky. In 2005, it was offered for sale at $1.2 million.