When buying Colored Gemstones, it is important to consider the following:-
- Is the stone Natural or Synthetic?
- Is the color natural?
- Is the color evenly distributed throughout the stone?
- Are there any noticeable inclusions?
- Do they affect the durability of the stone?
- How well is the gemstone proportioned?
- If the gemstone possesses “Optical Phenomena”, is it well defined?
- What is the weight of the gemstone?
- How do I care for the gemstone?
Evaluating Colored Gemstones
There are “3” factors that determine quality.
- The most important factor in the evaluation of Colored Gemstones.
- The gemstone should be checked under normal daylight, fluorescent light and incandescent lighting noting any color change.
Color can be described in terms of:
- Hue: which is the dominant and secondary colors visible in a stone.
- Tone: which is the sensation of the depth or darkness of the color perceived by the eye.
- Saturation: which is the strength, purity, intensity of the hue present.
The human eye can discern 150 different hues. Basic hue names are: Red, Orange, Yellow, Green, Blue, Violet and Purple.
Modifying names are then used to describe hues that fall between these terms, such as: Bluish, Greenish, Orangy plus other modifiers such as: Slightly, Strongly.
The Tone is described using the G.I.A Tonal scale ranging from 0 (Colorless) to 10 (Black).
Saturation is assessed on a scale ranging from 0 (Neutral) to 6 (Vivid).
In the warm colors (i.e. Red, Orange, Yellow), lower levels of saturation will appear brownish.
In cool colors (i.e. Green, Blue, Violet or Purple), lower levels of saturation will appear grayish.
In some cases, when dealing with stones of lower saturation, the use of such modifiers as brownish or pinkish may be used.
Color Grading Overview
- Thoroughly clean the stone.
- Hold the stone by the girdle in the tweezers. Look at the stone face-up from about 18″ away in strong, diffused, daylight equivalent lighting (with the light 6 to 10 inches above the stone)
- Estimate the hue of the dominant and secondary colors.
- Estimate the tone.
- Estimate the saturation.
Should be checked first with the naked eye, then with 10X magnification noting any obvious inclusions or blemishes.
Gemstones are divided into 3 clarity types:
- Type 1: Gemstones that are expected to form without inclusions (i.e Aquamarine). Even minor inclusions will detract from their desirability.
- Type II: Form with moderate inclusions (i.e. Ruby and Sapphire).
- Type III: Always found with inclusions (i.e Emerald)
Five factors to consider when “Clarity Grading”:
- Nature of the inclusion. (v) Position.
- Color and Relief.
|Morganite (Pink Beryl)||Orange|
|Alexandrite (Chrysoberyl)||Blue, Orange, Yellow Parti-colored (except watermelon)|
|Citrine, Amethyst, Ametrine||Green, Orange, Red & Yellow.|
|Emerald||Red or Pink Tourmaline or Watermelon Tourmaline|
- Clean the stone.
- Hold the stone by the girdle in tweezers or stone holder.
- Look at the stone with the naked eye in the face-up position.
- Turn the stone to view from all angles.
- Look at the stone under 10X with darkfield illumination.
- Turn the stone to view from all angles.
- Use an overhead light to check for surface blemishes.
- Assign a clarity grade.
Includes the proportions and finish of a gemstone.
May also refer to the shape or design in which the stone is cut.
The more expensive the rough, the less likely it is to be well cut due to weight retention.
- Practicality of outline and dimensions (including the ease in which the stone can be mounted and the presence of excess weight that does not contribute to beauty).
Evaluation of the skill and care used in the fashioning process and the gemstones overall surface condition.
- Face-up Outline
- Balance: The equality between corresponding parts of the stone in the face-up position.
- Length to Width Ratio: The relationship between the length and the width of a stone.
Shape Preferred Acceptable
Rectangular 1.50 – 1.75 1.33 – 1.50 1.75 – 2.00
Marquise 1.75 – 2.25 1.66 – 1.75 2.25 – 2.50
Oval 1.33 – 1.66 1.30 – 1.33 1.66 – 1.75
Pear 1.50 – 1.75 1.30 – 1.50 1.75 – 2.00
Heart 1.00 1.00 – 1.25
Appeal: The pleasing quality of the stone face-up outline.
Examples: Misshapen lobes on a Heart Shape.
Undefined point on a Pear Shape.
Flathead and high shoulders on a pear shape.
BRILLIANCE: Light return caused by internal and external reflections.
Two Factors that diminish brilliance:
- Windowing: A washed out area generally seen through the table caused by “light leakage”.
- Extinction: Areas of darkness visible through the crown.
Most stones return 65 to 90% of the light.
Dark stones and ones with extreme proportion variances may result in light return below 30%.
- Table size: 55 to 65%
- Girdle Thickness: Should be of medium thickness, thicker towards the points to prevent breakage.
- Total Depth: 60 to 75%
- Crown Height / Pavilion Depth Ratio: 1 : 3 (25% crown to 75% pavilion) or 1 : 2 (33% crown to67% pavilion)
- Bulge Factor: Minimal bulge is acceptable in Step-Cuts due to the slight changes in angle necessary from one row of facets to the next.
- Balance or Symmetry: The equality between the corresponding parts when viewed from the side or the end.
- Polish: Surface characteristics not included under “Clarity”. As a result of the polishing process or wear.
- Symmetry: consistency of the shape, positioning and arrangement of facets and the balance of the overall facet design.