The Kinds of Opals
Every opal is unique in appearance. Opals contain fireworks of every possible color dancing inside them. The flashes of color that make opal such an interesting stone are referred to as play of color. Opals will vary in body color, which is the background color of the stone. White body color is most common. The most valued body color is gray to black, with black considered best because it emphasizes and accentuates the play of color. Fire opal is material with yellow, orange, or red body color. In rare cases, opals will have green or blue body colors.
|Crystal Opal, 63.60 carats, Australia
(Photo by ICA/Bart Curren)
Combining body color and play of color we are faced with infinite possibilities. Pricing will be complex! Size of the opal will also be a factor for valuing; larger stones are rare so price per carat will accelerate accordingly.
The physical structure of opal is also unique; tiny spheres of silicon dioxide form a grid; between the spheres is water. The play of color occurs when the light refracts off different layers of the spheres of silicon, which acts like a diffraction grating. This structure is a fragile one, opals are quite brittle and should be set with care and in protective settings. Also the water that is suspended can dry out leaving the material cracked or crazed. Opal stored in water or glycerin will not craze, but opal stored in water all the time is difficult to wear! Before cutting opal a knowledgeable cutter will allow the rough material to dry out to stabilize it. That does not guarantee that opal will not be subject to further drying out at a later time.
|Colors:||Colorless, white, yellow, orange, red, green, blue, brown, gray and black|
|R.I.:||1.37 – 1.47|
|S.G.:||1.42 – 2.25|
|Treatment:||Occasionally stabilized, dyed, oiled and coated|
|Hardness:||5 1/2 – 6 1/2|
|Localities:||Australia, Brazil, Mexico, Honduras, Czechoslovakia and Nevada|
|Price:||Low to expensive|
|Common shapes:||Opals with play of color are cut into cabochons, any shape. Fire opal without play of color is faceted into any shape|
Play of Color
The “play of color” is also critical in the valuing opal. The intensity of the play of color and the extent to which it covers the opal’s surface are the two important factors to consider. The patterns of play of color are described as: pinpoint, tiny dots of color spread through the stone; harlequin, angular patches of color connected in a quilt-like pattern; or flash, moving spots of color that appear and disappear as the opal is moved. Opals with dominant red flashes of color will be valued more highly than opals displaying other colors if all other factors are equal.
If you are an opal buyer, you have many choices of color, quality and price ranges. A white or milky body color is the most common and the least expensive. Crystal opal is the term applied when the body color is colorless and transparent. Play of color creates the beauty in white bodied opal. Better quality stones will have intense play of color that covers the surface of the stone. Be sure and move the stone around and view it from various angles; the play of color is vary as it’s turned. Fine quality small white opals, that are under one carat will fall in the low price per carat range. Larger white opals with good play of color will fall into the moderate price per carat range. Exceptional large white opal can be expensive.
|(Black Opal. Photo by ICA/Bart Curren)|
Natural black opal will command the highest prices. Some stones will have dark gray body color but will still be called black opal. The darker the body color, the more valuable the stone will be. The play of color is important, of course. The finest “black opal” comes from Australia; it is mined at Lightning Ridge and Mintabie.
Opal can be dyed to make it black. Buying a black opal is another time when it is a good idea to ask a receipt specifying the natural origin of the body color. Buy black opal as a loose gem, or if it is mounted make sure the back of the stone is visible. Doublets are easily spotted when the back is visible. If the opal is set in a closed back mounting, you cannot tell if it is truly a black opal or a doublet or composite stone. Fine black opal with good play of color is the most expensive of the opals.
|(Fire Opals, Mexico. Photo by ICA/Bart Curren)|
True fire opal may have play of color or may not, but it always has a vivid orange-red to yellow body color. Fire opal is found in the brightest orange of any gemstone! Fire opal is mined commercially in Mexico. The material with play of color is highly unstable when it is first mined. The few pieces that do not craze are truly remarkable. Play of color will vary as will body color, but the most intense body color with the surface well covered with play of color will sell for the most. Fire opal without play of color is faceted and is a unique stone of its own. In this material the more transparent it is the more valuable. As the material approaches a bright red, the price goes up. The lovely yellow and orange stones sell for less.
Opals with Matrix
Opal is sometimes cut with pieces of matrix, the host rock that contains the opal. This strengthens the opal a bit; it also may lend a more interesting appearing, darker body color to show off the play of color. Stones that are more matrix than opal are called boulder opal. Boulder opal is usually sold by the piece rather than by the carat; fine pieces will be expensive.
Opal can form in organic materials such as skeleton and tree limbs which creates a pseudomorph. These are fossilized opals and can be very vivid in their play of color. Good examples of opal fossils are rare and may be difficult to find to buy.
Treatment of Opal
As mentioned earlier, opal can be dyed to produce a black body color. Material that is crazed can be oiled to hide the cracks. Material can be stabilized with plastics or resins to prevent crazing. Detection of these factors must be left to experts. When you are buying you opal have a detailed description written on the invoice.
Doublets and Triplets
Doublets are stones that have been coated on the back generally to give it the appearance of having a black body color. Triplets are composite stones that have a thin slice of opal sandwiched in between a colorless quartz top and black coated bottom. Doublets and triplets are created to imitate black opal, but they will be considerably less than an untreated black opal. They are generally sold by the piece rather than by the carat and will fall into the low and moderate price ranges. Triplets will wear better than solid opal because the quartz top has greater hardness than the opal and is not prone to chipping or cracking.
Care of Opals
Opals are one of the most fragile of gemstones. Choose mountings that protect the stone from all possible blows. Store opal jewelry separately to avoid scratching. Do not put opals in ultrasonic cleaners or expose them to rapid changes in temperature. Opals will not enjoy a plunge into dishwater! To clean an opal scratch off any excess grime with a wooden toothpick and soak the opal in a warm ammonia and water solution for a few minutes. With adequate precautions, you should be able to enjoy your opal for many years.