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In South Australia Opal was first discovered at Andamooka during the 1930’s. The first Andamooka Opal was discovered by Sam Brookes and Roy Sheppard. Caught in a thunderstorm they sheltered under a tree when a pretty rock caught the corner of their eye. Smaller deposits were exposed afterwards including Lambina during 1930 and Stuart Creek in 1947. Opal at Andamooka exists in the thin marine ‘Bulldog Shale’ which is the significant portion of the Marree Subgroup of the Early Cretaceous period.
Andamooka Opal is also known as the “Queen’s Opal”. It was cut and set into an 18 carat palladium choker band in 1954. It was presented to Queen Elizabeth II on the event of her first authorized state trip to Australia. Nowadays the necklace and earrings are considered a significant part of Queen Elizabeth II’s special crown jewels. The final Opal weighed 203 carats.
Discovered during 1930, Andamooka is considered a one of Australia’s most prominent opal fields. It has generated some of the most attractive pieces or sets of opal such as “The Andamooka Matrix Opal”, which is also referred as “The Queen’s Opal” mentioned above. The word Andamooka is derived from an Aboriginal name, meaning ‘Large Waterhole’. It is situated 600 kilometres north of Adelaide, and is the only Australian city where none of the streets are named.
The Andamooka opal mines are well-known for producing light opal, which compromises white, crystal opal, it is sometimes referred as “milk” opal.
Matrix Opal is a permeable opal, which originates from Andamooka, South Australia. A variety of boulder opal also appears in Andamooka. Most of the rock in the surrounding area is quartzite and so sometime the Opal forms in between this very ha rd rock. These are referred as the “painted ladies”.
Andamooka matrix opal is regularly treated to improve the colour by creating a black body using black carbon. This treatment involves a chemical treatment that is applied to the entire stone. When it comes out of the earth, it can look quite pasty, but by treating it by means of a carbon dye method it ultimately looks like a more appealing Opal.
Once it is totally treated, it is known as Andamooka treated matrix opal. With a rich solution of sugar with water, it is usually followed by a phase of simmering in a bath with a resolute sulphuric acid. The sugars pierce in the limestone matrix, and then the acid cooks it and that turns the white sugar around the pockets of opal into a black carbon. The outcome is that the darkened matrix encloses the pockets of opal, which does not diffract light. Some miners took the originals and sold them overseas as Lightning Ridge Black opal. After the scam was revealed, there developed a time when no one would purchase treated matrix. But the remarkable colours and rich black matrix of the best gemstones actually does present the attributes that make the Lightning Ridge black opal famous. Treatment of these matrix stones involves a moderately simple method of processing the stones.
Dinosaur fossils and bones have been found in Andamooka and many are now in museums. Australian dinosaurs are from 100 to 240 million years old which puts them in the Jurassic and Cretaceous periods. Many countries have dinosaur bones but only in Australia have opalized dinosaurs been found as opalized fossil.
Matrix opal can be quite reasonably priced, which is different with the authentic black opal. For the buyers, they should be aware with some opal dealer whether you are purchasing an authentic black or Andamooka matrix opal stones.
If the matrix was quite dense you should now be able to polish the stone. It is better not to use cerium oxide or tin oxide as the polishing compound can get into the pores and be seen as tiny white spots which can mar the surface. Use a leather lap with 50,000 grade diamond for the polishing.
Extra care needs to be taken not to apply too much pressure as this will generate a lot of heat and could crack the stone. The more porous matrix may not polish at all. If this is the case the surface can be covered with liquid glass to produce a highly polished surface. I will discuss this treatment in a different post.
Don’t forget to dispose the acid carefully. Add the acid to water very slowly and then bury it in the garden when it is very diluted. Do not add water to the acid as it will splutter and could easily splash onto you causing harm. If it does, liberally wash the affected area with water. If anyone has any comments on this treatment or any alternative treatments please feel free to comment. This process had worked for me and have some very beautiful stones which look very much like quality black opal and have cost very little in comparison.
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Also you can boil your stones in sugar water, then dry your stones on low in an oven for half hour, then soak in vegetable oil over night and then wrap them in alfoil and place on an open flame until all the oil is burnt off (you will know when done as the flames stop). Then your ready to epoxy and sand to polish. Black shoe polish applied to the finished stone also helps the shine and might cover fine scratches apparently. Pretty drawn out process but well worth it if you master the epoxying/polishing process which I have not so I just buy mine from a reliable source at a very good price. Peace to the world. Sincerely.