The Australian opal - The national gemstone of Australia. Australia produces Black, semi Black, Light opals and Boulder opals. Most of them are sedimentary, non-hydrophane opal.
110 million years ago, Australia was divided by ancient inland sea! as a result opalised marine fossils and Dinosaur fossils are found.
It's believed that perhaps 30 million years ago, lots of silica was released into a solution（a solution of silicon dioxide and water）which filled cracks in the rocks, layers in clay, and even some fossils. Some of them became precious opals. It takes about 5 to 6 million years to make a 1 cm opal to mature.That is why opal is so scarce.
Australian Opal Types:
• Black opal: recognisably looks black – if neither the background potch nor the face background look black, it is not a black opal. The black is significant because this is what helps the bright colours stand out and makes the stone look bright.
• Semi black opal: dark grey potch backing or grey background in the face.
• Light Opal or White Opal: Light base not transparent or semi transparent
• Boulder Opal:Precious opal sits on the top of the host rock( seam boulder opal)or mix with host rock(matrix boulder opal).
• Cystal opal: Can be any of the above. In Australia, any opal that is transparent or semi-transparent is considered to be a crystal opal. If it is clear like glass with flecks of colour it is often called a jelly opal, though sometimes this term is used to describe transparent crystal opal that has body colour but no real pattern.
Opal Mining Region in Australia:
• Nsw Lightning Ridge (the home of black opal), white cliffs
• The South Australian fields Coober Pedy, Mintabie(some blk opals）, Lambina and Andamooka (the main sources of light base opal) and
• The Queensland fields - yowah,winton,koroit… too many to list - where boulder opal originates.
In Australia, the major black opal producing fields are around Lightning Ridge in New South Wales.
The most valuable black opals are red or multicolour on black, which are very difficult to find. Such stones are always cut on the fields and sell immediately.
Green on black opals are getting more and more popular now especially on the Asian market.
Apart from seam opals, Lightning Ridge opals are usually found as nobbies - small blocks, pillows, spheres or hat shaped stones ranging from around 1-5 cm across. The stones usually have a light grey appearance when found due to a thin outer layer of grey potch. When stones are cracked open, they reveal black potch inside along with any colour bar. The difference between the outside appearance of a nobby and its inner fire and black colour when snipped or ground can be quite dramatic. Consequently, it is not uncommon for good stones to be missed (and thrown on the driveway as scraps)!
Opals from Lightning Ridge are often considered to be the best and brightest in the world.
The history of opal mining in Australia:
1877 – Mining of igneous rocks in search of opals, begins in central western New South Wales, at Rocky Bridge Creek, at the Abercrombie River.
1881 – In the far west of New South Wales, opal is found in Milparinka.
1884 – White Cliffs, also in New South Wales, is discovered to have opal buried in sedimentary rock.
1889 - Further mining and excavation reveals that White Cliffs also contains precious opal.
1880 - 1891– Opal is found in and around Lightning Ridge, also known as Wallangulla. These opals are found in sedimentary rock.
1890 – Mining operations in White Cliffs to extract precious opal from the area.
1896 - Near White Cliffs, opal is discovered in Grenville-Bunker Field, and Purnanga.
1897 – Tooraweenah is discovered to contain opals buried in igneous rock.
1901 – on the north coast of New South Wales, opals are found at Tintenbar.
1901/1902 – Jack Murray begins mining opals at Lightning Ridge. Some time later, Bathurst miner Charlie Nettleton also begins to mine at Lightning Ridge.
1903 – Charlie Nettleton sells a parcel of gems from Lightning Ridge, for merely $30.
1908 – Opal mining begins nearby Lightning Ridge, at the Grawlin-Sheepyard Field. Opals fields in the area become increasingly important.
1919 – Tintenbar is mined for opals until 1922.
1920 – Opal is discovered in Newfield.
1920 – Landmark studies by the Geological Survey of New South Wales means that exploration for opals becomes better and more controlled.
1989 – Near Lightning Ridge, opal is found in what is now known as the Coocoran opal area.
1998-1999 – New South Wales is now a leading global producer of opal, and the value of production is valued at $44 million.