by Frank Farnsworth
Tucson Seminar Notes: Opal Evaluation and Valuation
You can download Frank's PowerPoint presentation here (4MB)
This presentation delivers the information to recognize opal types, their characteristics and their value factors. We will discuss current sources and mining activity. We will work to be better able to establish and deliver the correct value decision.
12th c.; lat. opalus; gr. opalion; sansk. Upala
Translucent mineral consisting of hydrated silica of variable color.
- Family: Hydrous form of silica
- Chemical composition: SiO2. n H2O
- Crystal system: Amorphous
- Hardness (Mohs scale): 5,5 - 6,5
- Specific gravity: 1,90 ±0,30
- Refractive index: 1,44 ±0,03
- Birefringence: None
- Optical character: Singly refractive
- Water content normally three to ten percent, but may be as high as twenty percent.
- Actually mined in what is now Slovakia. Known as the first place opals were found.
- On the 28th July 1994, opal was declared Australia's National Gemstone. Presently Australia supplies the world with more than 90 per cent of its precious opal. Some of the better know mining areas are:
- White Cliffs
- Lightning Ridge
- Coober Pedy
- Pedro Secundo in the Piaui State. Boi Morto is where most of the Brazilian opal is found. Very similar to the Australian material it is often difficult to discern from its Australian cousins. There have been small parcels sold to Chinese cutters over the years and then cut and often sold as Australian material.
- Okanogan Valley, BC has volcanic deposit of opal.
- Often referred to as "Chocolate" opal due to a brown body color. This opal has been on the market only for about 17 years. As of early 2008, another opal deposit has been found , Welo Province, Which has produced a large quantity of precious opal. Many of the Opals are "Hydropane" that is the absorb water and the body color will become clear. This may lead to some issues of contamination by body lotions and liquids over time. The material seems to be stable but many dealers are still afraid of the material. Good article in Gems and Gemology Summer 2010 edition.
- Actually from Java, this opal has larger silicon spheres resulting in more red color. However much of the material is unstable.
- The oldest mine in Queretaro is Santa Maria del Iris this mine was opened around 1870 and has been reopened at least 28 times. At the moment there are about a 100 mines in the regions around Queretaro, but most of them are now closed. The best quality of opals came from Santa Maria del Iris. The important deposits in the state of Jalisco were not discovered until the late 1950's. In the year of 1957, Alfonso Ramirez (of Queretaro) accidentally discovered the first opal mine in Jalisco - La Unica, located on the outer area of the volcano of Tequila, near the Huitzicilapan farm in Magdalena. By 1960 there were around 500 known opal mines in this region alone.
- Blue "Common" Opal. Peruvian blue opal is relatively rare and is found in the Andes mountains near San Patricio, Peru. It is a very translucent stone with color similar to the Caribbean Sea. Depending on how the stone is cut it can be clear, scenic (showing varying degrees of color) or dendritic which has black fern like inclusions. This is the national stone of Peru and, unfortunately, in recent years dyed stones have been mass marketed by unreputable dealers as Peruvian Blue Opal. If the blue color is uneven you are probably looking at a dyed stone. Dyed stones are quite common and sell very cheaply at gem shows and on internet auctions.
- Virgin Valley, Nevada, SE Oregon, Idaho, near Yellowstone Park all have small productions of common and precious opal.
- Black Opal, Dark Opal and Light Opal: Material that is N1 to N4 is considered Black, N5 to N6 as Dark Opal and N7 to N9 as Light opal (N9 can be referred to as "White Opal"
- Boulder opal: Natural opal formed on Ironstone. "A natural doublet" The body color or tone is usually due to the Ironstone.
- Boulder Matrix opal: Boulder opal with Ironstone matrix interceding with the opal.
- Composite Opals: Doublets and Triplets.
- Treated matrix opals: Small specks of opal formed within the sandstone. Often the sandstone is treated with sulfuric acid and sugar to darken the sandstone allowing the opal to be more pronounced.
- Cabochons are most common but opal may be faceted, carved or cut into beads.
- Calibrated: Cut to exact size
- Free Size: Cut to regular shape but not calibrated.
- Free Form: Cut to maximize weight.
Composite or Assembled:
- Doublets: Usually cut with solid opal on the top and ironstone or another backing material glued to the back.
- Triplets: Clear crystal glass or quartz on the top and a thin slice of opal in the center with black glass or other backing material on the back.
Treatments or Enhancements:
Seldom are opals treated, however, we occasionally will see wax coatings on solid opals, resins used to fill crazing with limited success. Sugar and sulfuric acid treatments of Sandstone matrix opals.
We have identified many of the different types of Opal now we will discuss how opal values are determined.
Base Color: "Body tone"
A continuum from crystal clear body color to grey to solid Black.
Intensity of color: "Brilliance"
Intensity of fire! Brilliance is the most important value factor in opal. How bright is the "fire" in the opal? This is the most difficult attribute to describe and to remember. We do not have language to help us explain how intense or bright the colors are.
Here we describe how translucent the body color is, from crystal clear to solid.
A continuum from Pin fire to Broad Flash with Harlequin in the center and the most desired.
Dominant Diffracted colors:
Play of color: What spectral colors do we see when viewing the opal at all angles. Since opal breaks spectral light the colors can be of any make up. We do know that if the silica spheres are small (less than .20 micron) the diffracted light will be blue, at .25 microns we get green color and at .32 we get red color. Watch for "directional play of color". This will greatly influence the value of the opal.
Thickness of the Color Bar:
Sometimes the thickness of the cut stone is due to the cutter leaving common Opal "potch" on the back of the stone. A thicker color band adds value.
Opal Fossils and Specimens:
Opal has replaced the bone structure in prehistoric dinosaurs, sea animals and woods. These rare specimens may have a unique value far beyond the "Opal Value" and may need to be reviewed with Scientists who study fossils. Opalized Belemnite Species: Neohilobites Sp.(The fossil is the main body tube of an animal similar to a present day Cuttle Fish) Cretaceous Age. Also, Opal Clamshells are very common.
- Hydrophane is a type of opal that will re-absorb water as it is very porous and then dry to a chalky white.
- Opal enhydrites are where ancient water and or air is trapped in the opal are very rare.
- Star and Cats Eye opals may rarely occur due to exact faulting of the Silica layers.
Identifying crazing, cracking and dehydration in opals:
Since opals are formed with water content from 3% to as much as 20% the natural drying of the material can cause fracturing (crazing) to develop within the opal. Keeping opal in a humid environment can lessen the occurrence of crazing. Once the crazing occurs little can be done to salvage the stone. Direct sunlight through an outside display window can magnify the heat and create crazing. Crazing is easily identified by candling the stone. Often you will see a spider web effect.
Cracks, on the other hand, may be caused by a blow to the surface of the stone and a careful examination will usually reveal a point of impact.
Dehydration is another possible issue created by a lack of humidity or too great of water content in the material.
Created opal is primarily being supplied to the Asian manufacturers by Kyocera in Japan. They claim that the chemical properties are the same as natural opal.
Part Two: Opal Valuation
Now that we have discussed origins, the influencing factors and cutting choices it is time to turn our attention to how these factors influence value of the finished gem. Trying to determine the price of very rare items is almost impossible hence we want to start with the least rare:
We have developed a grading system from N4 to N0 (not to be confused with the body tone index used by the Geological Society of America) This grading system is based on the intensity of brilliance of color.
Here is a table referencing the approximate retail value of Light opal based on this grading system and this date:
|Under 1 carat:||$40/ct||$60/ct||$120/ct||$200/ct||$300/ct|
|1 to 5 carats:||$50/ct||$100/ct||$220/ct||$1000/ct||$2000/ct|
|Over 5 carats:||$60/ct||$150/ct||$300/ct||$1500/ct||$2500/ct|
Prices are for calibrated well cut light opals from white to crystal in body color. If the stone is free size (not calibrated) but well cut the price should be 20% less than these prices. If cut in free form shape the price should be 30% to 40% less. Grey based material is normally priced 10 to 30% less as it is less desirable. Prices are retail based on triple key.
We have developed a grading system for free form cut doublets as follows:
|From 1 to 10 carats:||$25/ct||$40/ct||$100/ct||$200/ct||$300/ct|
Prices are for well cut and polished free form doublets with little to no inclusions. Calibrated doublets would cost approximately 20 to 30% more based on size. Stones over 10 carats are rare and need to be valued individually. Prices are retail based on triple key.
Boulders Matrix opals:
Here are some examples of pricing for matrix boulders opal. Prices are based on the amount of opal versus matrix and the intensity of the color in the opal.
Value must be adjusted for cutting and polishing and general interest of the stone. Boulder opals without matrix are rare and need to be valued individually using the Black Opal value as a guideline. Prices are based on triple key.
Currently the production of full color boulder opals is very low. Also, the boulder opals are very individual stones and replacement is very difficult. Knowing this it is best to acquaint ourselves with many samples and varieties to get an idea of what current market prices are. Boulder opals are normally priced per piece rather than per carat. Price range can be from $300 up per piece based on intensity of color, number of colors present, size, cutting, polish and interest of the piece.
Currently the production of Black Opals is very low. Due to this any insurance appraisals must be carefully reviewed as replacement stones are difficult and often impossible to find. Overall guidelines for Blacks (opals in the N1 to N4 body tones) range from $150 per ct for predominately blue color low intensity blacks to $15,000 per carat for fine black crystal with multi color or predominate red color well cut free size.
Special Thank you to Andrew and Damien Cody, National Opal Collection and Cody Opals, Melbourne and Sydney, Australia for the use of there slides and material from "The Opal Story" "A Guidebook"
- Cody, Andrew and Damien (2008) The Opal Story A Guidebook, ISBN No. 978-0-9805987-0-4
- Leechman, Frank (1961) The Opal Book, Publisher: Ure Smith, Pty., Ltd, Sydney, Australia
- Ward, Fred, Opals, ISBN No. 1-887651-04-7
- Gems and Gemology Summer 2010 Issue by B. Rondeau, Emmanuel Fritsch and others
You can download Frank's PowerPoint presentation here (4MB)