Opal got its name from the greek word "Opalus" which meant "to see a change in color." Opals were first made popular by the Romans and opal in these ancient times came from Cernowitz (Vorosvagas), a mountainous region in Hungary, now part of Slovakia where hundreds of men mined the stone.
Hungarian opal is a milky (white) type of opal, far inferior in quality compared to Australian opals in fire and brilliance. I have personally visited the region where the Hungarian opal was mined and was able to buy a few pieces of this opal from local people who still enter these ancient mines deep inside the mountain.
The Romans considered opal to be a symbol of hope, an appropriate attribute for a gem with a rainbow locked within it.
The Arabs believed opals fell from heaven in flashes of lightning, thus acquiring their fiery colors.
An example of Hungarian Opal seen on this image, found in the 17th century in Hungary, measuring 130mm x 70 mm x 70 mm and weighing 594 gramms. Displayed in the Museum of Natural History Vienna, Austria
History of Opal Mining in Australia
Australia is the 'real' home of Opal today and the most important supplier of Black Opals, Crystal Opals and Boulder Opal. Almost 95 per cent of all Opals come from Australian mines.
In 1849 the first pieces of Opal were accidentally found on an Australian cattle station called Tarravilla . Opal mining started in 1890 at White Cliffs.
Opal was discovered in the 1870's in Central Queensland. Boulder Opal is found in this vast area that is appr. 1000 kms long and 300 kms wide. Important mining centres include the towns of Yowah, Quilpie, Eromanga and Jundah.
In 1901 opal mining begins at Lightning Ridge. Lightning Ridge is the home of Black Opal, the most precious variety of all opals.
In 1908 opal mining begins at the Grawin-Sheepyard Field in the Lightning Ridge area, increasing the importance of the opal fields in the district.
What is opal?
Opal is a true precious stone which occurs in many varied forms. Opal is amorphous silica with a water content varying from one to twenty percent, depending on the porosity and degree of hydration. Precious opal usually contains from six to ten percent water. The chemical formula for opal is SiO2·nH2O
What causes the play of color and different patterns in opal?
In the mid 1960's a group of scientists at the Commonwealth Scientific and Industrial Research Organization (Australia) solved this mystery and put an end to all previous theories.
By using a newly developed electron microscope which magnified 30,000 times they found opal consisted of tiny spheres of silica ranging in size from 0.00005 mm to 0.00004 mm. By comparing common opal (colorless potch) to precious opal they discovered that precious opal was composed of very constant sized spheres in a tightly packed uniform pattern. In common potch the spheres are jumbled together in no particular pattern and are of different sizes.
Color is produced by light entering the opal and being broken up into colors by the spheres and spaces between them. It follows that the space between the spheres must he uniform since the spheres are uniform. This light phenomenon is called diffraction.
The size or the spheres determines the color one sees. Large spheres (0.00003 mm) (equals one millionth of an inch) produce reds. Blue and greens are produced by medium size spheres. Small spheres produce violet.
There is usually not one constant pattern over the whole stone. There are breaks in the size of the spheres or the orientation of the lines of spheres. These breaks in the alignment of the spheres account for the different patterns of opal.
The brightness of an opal is measured on a scale from 1 to 5. 1 being dull and 5 being brilliant. Only the brightest of gems deserve a 5, while most opals are between 3 and 4.5. When buying an opal, make sure the brightness is 3 or more on the scale.
The body tone of an opal is different to the play-of-colour displayed by precious opal. Body tone refers to the relative darkness or lightness of the opal, while ignoring its play-of-colour. This is assessed on a Scale of Body Tone . The boxes (below) comprising this scale, represent approximate values of body tone in equal intervals from black to white.This arrangement is in agreement with all known scales of tone used in colour science, and is well illustrated in the commercially available Rock-color Chart produced by the Geological Society of America.