Opal is unique among gemstones. It inspires the strongest of emotions. Few gem lovers are neutral about opal. One either loves opal or one fears it. The Romans loved opal and considered it good luck. The Australian aborigines, who associated it with the Rainbow Serpent, were fearful of it. Marbodius, a medieval Bishop of Rheims, declared it to be the patron stone of thieves and attributed to it the ability to make the wearer invisible. The father of William the Conqueror attributed all the bad luck of his house to the ownership of a single gem. Today many people still believe that the wearing of opal is bad luck unless it is one's birthstone.

The Roman historian Pliny identified India as the earliest source of gem opal. There are early references to mines at Poona, Bijapur and Sitibali. None of these sources has ever been proved. The only documented ancient source of opal is the volcanic deposit located southwest of the northern end of the Carpathian Mountains in what is now Slovakia. These deposits were formed by silica dissolved and deposited in cracks and veins of volcanic rhyolite, trachyte and basalt.

There is very little evidence that gem opal was known at all in the west prior to the Romans. No known written references predate Pliny. There are no extant examples of opal jewelry from European or Mediterranean cultures prior to Roman times. The word itself comes from the Latin "opalus", the Greek "opallion" is derived from the Latin. Modern discoveries of high-grade opal in Ethiopia has led to speculation that this may be the Romans Asian source. Opal from this source is, however, rather distinctive and more of the Mexican type. The most likely scenario is that beginning about 50 BC opal was traded from its Carpathian source down the Danube river to the Greek city of Corinth and from there to Rome itself.

Australia is the modern source of over 90% of the world's opal. Australian opal was first discovered in Queensland at Listoral Downs and at Springside in 1872. Commercial mining in Queensland peaked in the 1890's, declining sharply after 1895 due to drought. Deposits of opal were discovered at Whitecliff in 1890 and Lightning Ridge in 1902. The two major producing centers; Coober-- Pedy and Mintabe--were discovered in 1915 and 1919 respectively. Coober Pedy was the single largest producer until it was surpassed by Mintabe in the early 1990's. Coober Pedy is still the most important producer of commercial grade white-based opal.

Photo: R. W. Wise Open cut mining in the Australian outback. Here a bore is made by a Caldwell Drill in an area outside Opalton, Queensland. Mining is a hit or miss affair. If traces of color are found, one of the miners will be lowered into the hole with a flashlight.

STABILITY

Unlike most gemstones, opal, like glass, is non-crystalline. It is basically a silicate, 85-99% pure, with the chemical formula SiO2.nH2O.

Two important factors affect the stability of opal: toughness and water content. Opal rates 5 1/2-6 on the Moh's scale, although some stones from Brazil are close to 7. Opal is, therefore, somewhat prone to scratching and abrasion. Toughness is another matter; opal is rather brittle and subject to chipping. Gem Opal contains between 6-18% water. Water content is important because it directly relates to the stability of the gemstone. Opals with high water content are unstable and more likely to craze or crack

There are six main types of opal classified according to body color. Black opal has a dark gray to black body color. Additionally, the stone must be opaque when held to the light to be classified black opal. Semi-black has a dark body color but is translucent when held to the light.

Crystal opal has a dark to light almost transparent body color and looks like its name. White opal, the most common, has a translucent to opaque milky white body color. Mexican opal has a red to orangy-brown body color. Boulder opal may have any body color but will include ironstone matrix as part of the cut gemstone. Boulder opal has very low water content and may have some legitimate claim to the title of most durable opal.

JUDGING QUALITY

Color in gemstones is divided into three components: hue intensity (saturation) and tone. Hue, what we usually mean when we use the word "color” is the single most important factor in defining value in most gemstones. For example, the reddest red is the most important factor in the value equation of ruby. In opal it is the brightness or intensity of the given hue that is most important. In the words of one prominent dealer: "opal is like a light bulb, the brighter it is the better it is."

Once we dispose of intensity, hue is the next most important factor. There is a hierarchy of hues. It is generally agreed that red is the top color, followed by orange and ending with green. Here the agreement ends. Multicolor, an opal with three or more hues is the most desired. Richard Drucker author of "The Guide" a well respected gemstone price list, reserves his highest rating for a stone that is seventy-five percent red and containing two additional hues1.

Photo: R. Weber By far the rarest and most expensive color, “red opal”, a stone with fifty percent or more of a visually pure red play of color will command the highest price. In addition, these two stones show exceptional saturation.

ONE GLARING EXCEPTION:

Fire opal is graded differently from all other types. In this type of opal body color is visually dominant and is considered the key value factor. In fact, fire opal has little if any play of color. Dennis Schmelzeneach of Opal International suggests five most desirable qualities based on body color, ascending from least to most expensive; these are: light orange, medium orange, dark orange, orange/red and cherry or fire engine red. Other authorities, notably Paul Downing, consider orange/red to be the top color.

Pattern is an important determinant of value. Again, there is general agreement that harlequin, squarish blocks of color which resemble the patchwork costumes of the clown figures featured Picasso's early paintings is the most desirable. Pin-fire, a pattern composed of myriad pinpoints of color, a sort of Technicolor Milky Way, the least desirable. However, the question of pattern can be made needlessly complex. It is rather a question of composition. Pattern should be judged as if the gem were a fine abstract painting. The play of color should be pleasingly distributed across the face of the stone and appealing to the eye.

Photo: R. W. Wise Traces of color in rough boulder opal. This chunk of ironstone has been sliced on a saw.

THE CARE AND FEEDING
OF FINE OPAL:

Bring a few opal experts together or read a couple of books and one thing is certain; disagreement. Some people believe you should store opal in water others use glycerin. Both practices are common in the opal fields. Others keep their opals immersed in oil. While small amounts of oil rubbed into the surface of a wear-pitted stone may temporarily enhance the clarity of the stone, oil and water do not mix so oil immersion is not recommended. Glycerin is actually a drying agent and although its immediate effect may seem positive it may have a long-term negative effect on the gemstone.

Opal is brittle and must be protected when setting. Improper setting of opal in jewelry is a major reason why opal has developed such a bad reputation for durability. Prong setting of opal in rings is a common practice. It is also, unfortunately, a prescription for disaster. The exposed edges of prong set opal are much more likely to chip. Opal, especially fine stones, should always be bezel set.