text: Walter Bartram was prospecting in dusty terrain in Coober Pedy, about 750km (466 miles) north of Adelaide, in 1946 when he staked a claim to what became called the Fire of Australia.
  Although his family achieved success in opal trading, their greatest discovery has been seen rarely by the public.
  That has just changed.
  The 998g (35.2oz) opal, valued at nearly A$900,000 (Â£550,000; $680,000), is now on display in Adelaide's South Australian Museum.
  Still largely in its original condition, the opal's two polished faces reveal a kaleidoscope of colours from green to yellow to red.
  "When my father was alive, it was originally kept separately from all trading because it was such a significant piece," Alan Bartram told the BBC.
  "We decided we would retain that intention, and keep it as a significant and obviously excellent example of light opal from South Australia."
  The family has decided to pass it on for future generations to enjoy.
  The museum's director, Brian Oldman, said the opal's rarity should not be underestimated.
  "Opal of this quality can only be created under certain climate conditions," Mr Oldman said.
  "When our state's inland sea evaporated millions of years ago, it provided a unique silica-rich environment for the creation of precious opal. It is these exceptional conditions that created the Fire of Australia."
  A mining town for more than 100 years, Coober Pedy still draws people lured by the hope of striking it rich.
  "They're becoming more scarce because the overheads of mining now are getting to be so expensive - in fuel, explosives, machinery and living costs on the field," Mr Bartram said.
  "But South Australia supplies about 90% of the world's quality opals. There may be more major finds."
  Reporting by the BBC's Greg Dunlop
target: The world's finest uncut opal has mostly been kept in a safe deposit box since it was unearthed from the South Australian outback with a pick and shovel 70 years ago.