Buying Coloured Gemstones

Overview

When buying Coloured Gemstones, it is important to consider the following:-

1. Is the stone Natural or Synthetic ?
   
2. Is the colour natural ?
   
3. Is the colour evenly distributed throughout the stone ?
   
4. Are there any noticeable inclusions ?
   
5. Do they affect the durability of the stone ?
   
6. How well is the gemstone proportioned ?
   
7. If the gemstone possesses "Optical Phenomena", is it well defined ?
   
8. What is the weight of the gemstone ?
   
9. How do I care for the gemstone ?

Evaluating Coloured Gemstones

There are "3" factors that determine quality.

1. Colour
   
2. Clarity
   
3. Cut

Colour

 
1. The most important factor in the evaluation of Coloured Gemstones.
   
2. The gemstone should be checked under normal daylight, fluorescent light and incandescent lighting noting any colour change.

Colour can be described in terms of:

1. Hue: which is the dominant and secondary colours visible in a stone.
   
2. Tone: which is the sensation of the depth or darkness of the colour perceived by the eye.
   
3. Saturation: which is the strength, purity, intensity of the hue present.

Hue

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 in between these terms, such as: Bluish, Greenish, Orangy plus other modifiers such as: Slightly, Strongly.

Tone

Tone is described using the G.I.A Tonal scale ranging from 0 (Colourless) to 10 (Black).

Saturation

Saturation is assessed on a scale ranging from 0 (Neutral) to 6 (Vivid).

In warm colours (i.e Red, Orange, Yellow), lower levels of saturation will appear brownish.

In cool colours (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.

Colour Grading Overview

 
1. Thoroughly clean the stone.
   
2. 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)
   
3. Estimate the hue of the dominant and secondary colours.
   
4. Estimate the tone.
   
5. Estimate the saturation.

Clarity

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:

1. Type 1 : Gemstones that are expected to form without inclusions (i.e Aquamarine). Even minor inclusions will detract from their desirability.
   
2. Type II: Form with moderate inclusions (i.e Ruby and Sapphire).
   
3. Type III : Always found with inclusions (i.e Emerald)

Five factors to consider when "Clarity Grading" :

1. Nature of the inclusion. (v) Position.
   
2. Size.
   
3. Colour and Relief.
   
4. Number.

Clarity Classifications

TYPE I

Beryl	Topaz	Chrysoberyl
Aquamarine	Blue	Green
Green Beryl	Yellow	Yellow
Morganite (Pink Beryl)	Orange
Yellow Beryl	Pink
	Red
Quartz	Spodumene	Tourmaline
Smoky	Kunzite	Green
	Green
Zircon	Zoisite
Blue	Tanzanite

TYPE II

Andalusite	Tourmaline
Alexandrite (Chrysoberyl)	Blue, Orange, Yellow Parti-coloured (except watermelon)
All Corundum
All Garnets
Iolite	All Spinels
Quartz	Zircon
Citrine, Amethyst, Ametrine	Green, Orange, Red & Yellow.

TYPE III

1. Clean the stone.
   
2. Hold the stone by the girdle in tweezers or stone holder.
   
3. Look at the stone with the naked eye in the face-up position.
   
4. Turn the stone to view from all angles.
   
5. Look at the stone under 10X with darkfield illumination.
   
6. Turn the stone to view from all angles.
   
7. Use overhead light to check for surface blemishes.
   
8. Assign a clarity grade.

Cut

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.

Proportion Grading

Considers:  

1. Beauty
   
2. Appeal
   
3. 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).

Finish Grading

Consider:

Evaluation of the skill and care used in the fashioning process and the gems overall surface condition.

Proportions

Three Steps

1. Face-up Outline
   
2. Brilliance
   
3. Profile

Face-Up Outline 

1. Balance : The equality between corresponding parts of the stone in the face-up position.
   
2. 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 stones face-up outline.

Examples : Misshapen lobes on a Heart Shape.

Undefined point on a Pear Shape.

Flat head and high shoulders on a pear shape.

BRILLIANCE : Light return caused by internal and external reflections.

Two Factors that diminish brilliance:

1. Windowing : A washed out area generally seen through the table caused by "light leakage".
   
2. 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%.

Profile

 
1. Table size : 55 to 65%
   
2. Girdle Thickness: Should be of medium thickness, thicker towards the points to prevent breakage.
   
3. Total Depth : 60 to 75%
   
4. Crown Height / Pavilion Depth Ratio : 1 : 3 (25% crown to 75% pavilion) or 1 : 2 (33% crown to67% pavilion)
   
5. 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.
   
6. Balance or Symmetry : The equality between the corresponding parts when viewed from the side or the end.

Finish 

1. Polish :  Surface characteristics not included under "Clarity". as a result of the polishing process or wear.
   
2. Symmetry : consistency of the shape, positioning and arrangement of facets and the balance of the overall facet design.