Colored Stone Grading - Opals
Opals, the most commercially important phenomenal gems, are graded by the same general principles as transparent faceted stones. However, they possess many unique characteristics which require additional explanation.
Play of Color
The most significant portion of the value of an opal derives from the quality of its play-of-color, a unique interaction between internal structure and light, which results in an intense display of spectral colors arranged in patterns which can change and shift dramatically as the stone or viewer moves. Play-of-color is graded by intensity, dominant colors, pattern and coverage. Opal evaluation is very subjective, and members of the trade often disagree about the significance of various characteristics.Type
We grade an opal by first identifying its type. Opal is a diverse gemstone — some types have play-of-color, some do not. Opal with play-of-color is generally the most beautiful and valuable. It is sometimes referred to as "Precious Opal," as opposed to "Common Opal" which has no play-of-color and is often opaque or translucent rather than transparent.
Opals with play-of-color are grouped by background color or structure as follows:
- Black - very dark black, brown, or gray body color or background matrix.
- Gray - medium to light tones of gray body color or background matrix.
- Crystal - highly transparent body, no color.
- Semi-crystal - somewhat transparent body, no color.
- White - white base or body color, the type of opal with which most people are probably familiar.
- Boulder - crystal opal occuring in veins or patches within or on an ironstone or sandstone matrix.
- Fire — transparent variety that has yellow to red body color and may or may not have
play-of-color, also known as "Mexican Fire Opal." - Assembled - processed material where a very thin slice of opal is affixed onto a dark background, may also have a glass or crystal dome affixed on top. While they still contain some precious material, these stones are much less valuable than natural opals.
- Treated - material that has been enhanced in order to improve the body color or play-of-color.
- Synthetic - any of several man-made, opal-like materials.
The primary source of precious opal is Australia. The Aussie opal dealers and miners may also use the following type designations, among others:
- Knobby - specific formation only found in Lightning Ridge, featuring a crystal-type center with matrix on the outsides.
- Crystal Black
- Semi-Black
- Jelly
- Water
- Ironstone - based on locale, with subtypes from Yowah, Queensland, or Koroit.
Pattern
Describing the color patterns of the play-of-color in precious opal is perhaps the most subjective area of grading. Some opals may display combinations or more than one pattern. The desirability of a particular pattern is to some degree based on rarity, as in the harlequin patterns, but it's also a matter of personal preference. Many patterns are truly unique and call forth the most poetic and passionate descriptions, however we can categorize them into three basic groups, with additional variations.
- Pinfire — typically small points or dots of color that cover an area, the type of opal with which most people are probably familiar. Variations: Chaff, Twinkle, Straw.
- Flash — larger areas of play-of-color that often shift or change color with movement. Variations: Broad Flash, Rolling Flash, Exploding Flash, Ribbon, Rainbow.
- Harlequin — large, distinct, and usually rectangular patches of play-of-color with edges that touch each other. Variations: Checkerboard, Flag, Fish Scale, Flagstone.
Brightness
The intensity of the play-of-color in precious opal is rated on the following scale:
- Level 1 - Faint. Play-of-color only under bright light, almost non-existent.
- Level 2 - Dull. Shows some color under low light, but is dull even under the grading lamp or in indirect sunlight.
- Level 3 - Bright. Shows fair color under low light, but pleasant and bright under the grading lamp or in indirect sunlight.
- Level 4 - Very bright. Shows good color under low light, sharp crisp color under the grading lamp or in indirect sunlight.
- Level 5 - Brilliant. Shows exceptionally bright crisp color under the grading lamp or in indirect sunlight. May show even brighter in subdued light.
Coverage
Voids and dead spots in the play-of-color are less desirable. We rate the coverage of the face up view of the opal as a percentage, 100% being the ideal.
Colors
Bright spectrum colors are the most desirable. The more pure and vibrant hues are better. Because the colors in play-of-color are directly related to the size of certain structures within the opal, some colors are more rare in nature, red in particular. Colors are generally ranked in order of value as follows: red, violet, orange, yellow, blue, and green.
Directionality
In addition to rating the brightness, we also consider the directionality and consistency of the play-of-color. This will be described with a narrative. Strongly directional stones are generally less desirable. A stone must be consistently bright in all directions to be considered top quality.
Cut
Opal is usually cut as a cabochon, although some of the more transaparent types (especially fire opal) may be faceted. Many opals are cut with the play-of-color layer positioned over a background of matrix or colorless material called potch. If the matrix or potch is dark, this can emphasize and show the play-of-color to better advantage.
Boulder opal often occurs as very thin seams of opal in an ironstone matrix, which may be split open to reveal the precious opal. These stones are often finished by simply polishing the split surface, leaving the natural contours, or in some cases where the color layer very thin, not polishing the color layer at all and just trimming the outline and back. Because the opal layers are often very thin on boulder types, it is not uncommon to have matrix exposed thru the color layer to the face-up view—this is generally a negative.
Evaluating the shape of an opal is often purely subjective. The general face up rules for proportion governs calibrated commercial material. Thickness of material and suitability for setting is an important consideration since opal is a relatively delicate gemstone. Baroque stones will be graded by overall face up appeal and suitability to the play-of-color present. Excessive weight retention is a negative.
We describe the overall cut of an opal with the following terms:
- Excellent
- Very Good
- Good
- Fair
- Poor
Polish
We use similar standards for grading the polish of an opal. Because opal is a heat-sensitive gemstone, the craftsmanship and skill of the lapidary is very important. We describe the polish with the following terms:
- Excellent
- Very Good
- Good
- Fair
- Poor
Quality Grade
On a scale of 1 – 10, we rate the combined effects of pattern, coverage and directionality, brightness, colors, and cut, as well as rarity. Since play-of-color is the dominant feature for grading, we start there and then apply adjustments for cut and polish. Cut is less important in grading opals than for transparent faceted gemstones. So, a stone graded 9 for play-of-color could have good cut grades of 6 and still be considered Extra Fine 9. A stone graded 8 for play-of-color, with excellent cut and polish could be graded overall as 8.5.
In our appraisal and grading practice, we follow the GIA Gemstone Color Description System and the general grading/pricing structure found in the Gem Guide wholesale gem pricing book published biannually by Richard Drucker, Gemworld International Inc., because we find it to be the most reliable and consistent source of market based price data available for colored gemstones. Since price follows grade, or vice versa, our quality grades will reflect our opinion of the position of a gem within the market place.
- 8 - 10: Extra Fine, the best of the best, rarely seen, and never cheap. Play-of-color is always brilliant, coverage 95%+, usually bright in all directions. Should be an interesting pattern. May have only minor defects in cut. These stones are often truly unique.
- 6 - 8: Fine. Play-of-color is always very bright, some parts brilliant, coverage 90%+, usually bright in most directions. May have minor to slight defects in cut. White base opals usually don’t exceed this category.
- 4 - 6: Good. Play-of-color is bright, some parts very, coverage 85%+, may be noticeably directional. May have slight defects in cut, should not be too thin.
- 1 - 4: Gemstones that fall into this category are what the trade calls "commercial". Can include stones with some bright play-of-color, 50-80% coverage, some dull areas face up. Low commercial may be very thin or otherwise poorly cut, dull, 20% coverage, and/or deficient in colors.