Opal at Andamooka is found in a narrow band of depths varying up to 600 mm. In this bed there are many boulders of strongly bonded quartzite of varying sizes.,Today, as you wander among the great piles of disturbed overburden which is mixed with the old opal level , you will come across many of these boulders. They are extremely hard and difficult to break.
The quartzite boulders often look like the above photograph but are not usually located on the top of a post as in the photo.
It is worthwhile looking carefully at them because some of them may contain opal. When the opal was being formed, the silica was deposited in any cracks and fissures in these quartzite boulders and sometimes this formed thin but brilliant pockets of gem opal. When these are broken along the opal filled cracks, the resulting opal faced fractured surfaces produce what is called a "Painted Lady". These can be very beautiful and sometimes very valuable especially if the opal is of gem quality.
Sometimes the opal is too thin to polish but on other occasions they can be polished and made into beautiful specimens.
I have been fortunate to find a few smaller examples of Painted Ladies and a few of these are shown in the photographs in this post. There is still a chance to find them on the many heaps of material left by the miners at Andamooka, so it is well worthwhile scouring these dumps.
A good website to see a number of specimens is Painted Ladies Opal.
Johno