For those who haven't seen it before the largest opal found is The Galaxy Opal. The Galaxy Opal was found in Brazil in the 1970's. Certified by the Guinness Book of records in 1992, as the largest ever polished Opal it weighs approximately 3,749 carats which is around 0.75kg.
The actual Opal that was found in the Boi Morto Mine which is in the North Eastern part of Brazil weighed just over 5,000 carats and was carved into a shape which was aimed at resembling a child's head. The full measurements are: 14 x 10.2 x 4.1cm. It was so much bigger than any other Opal it’s carats had to be estimated using a postal scale. A little bit more info here
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