History of Opal 

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. Opal Classification Brightness 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. Body Tone 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.

Opal Types

Black opal is the most rare and valuable of all opals. It is solid (natural) and generally found as a bar (or bars) of various colours in a dark body (black, blue, brown or grey). Some black opals have a complete rainbow of colours while others have deep blue-green hues.

 Body tone of black opals is from N1 to N4. Refer to the chart below.

 Black opal from Lightning Ridge on the left N2 in body tone and weighing 11.80cts, the opal in the middle is a black crystal opal Translucent, N4 in body tone weighing 1.10cts and the one on the right is a Black Opal N1 in body tone and weighing 1.90cts. 

There are also black crystal opals. While a true black opal displays sharp brilliant colours on a dark background, in contrast, the black crystal opal is translucent with no traces of black potch on it underside. Its colours are sharp and visible beneath the surface.

Dark Opal

Dark opal is the family of opal which shows a play-of-colour within or on a dark body tone, when viewed face-up and may be designated N5 or N6 using the Scale of Body Tone. See the chart below.

The opal on the left is a dark based opal N5 in body tone weighing 9.50cts, the one on the right is a N6 dark based opal, weighing 2.50cts.

Light opal is also solid (natural) opal. Lightning Ridge and Coober Pedy are the places where light opal is mined in Australia. Also found in Andamooka and Mintabie but these places hardly produce anymore.

A full range of colours swirls and flashes in the depths of a light opal. The background colour may be white for white opal or light blue for light crystal opal is translucent and shows colours sharp and visible below the surface.

A nice crystal opal, transparent weighing 1.10 cts.

Recommended site links for further information on opals

ICA - International Colored Gemstone Association  An informative page on Opals. The International Colored Gemstone Association is a non-profit association to represent the international gemstone industry.

The American Opal Society The American Opal Society is an non-profit organization, educational in nature, whose primary purpose is promoting interest and knowledge of this precious gem.