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Choosing a coloured stone

In a similar way to diamonds, coloured stones can be classified by the method of 4Cs : Colour, Clarity, Cut and Carats (weight).

However, there is, at the present, no price list setting the exact value of a stone.
Indeed, only the diamond has a monthly dollar trading value table (The Rapaport Diamond Report).

Accordingly, there are some basic rules to evaluate the quality of a stone. The real value is defined by the market.

The colour

This is by far the most important factor and the most difficult to judge.

A slight difference in color or saturation of color can greatly increase the value of a stone. We can describe the colour using the three following criteria:

The hue

The tone

The saturation

a. The hue :


This is the initial impression of the colour, that is to say:

        - The primary colors: yellow, blue, red.

        - The complementary colors: orange, green, purple.

           see color wheel below

    Example: We can define green in 3 ways: Green blue / green / yellowish green.


    b. The tone :

    Ranked from 1 to 10, a color can range from very light to very dark: grade 1 being transparent (white), grade 10 is black.

    In the world of colored stones, we generally use the tones 2-8, from "very clear" to "very dark".



    c. The saturation :

    Ranging from weak to strong, saturation describes the intensity of the color


Conclusion :



Each type of stone has its range values of hue, saturation and tone.

Within this range, only a small part is considered (by the market) as the most beautiful color, representing the highest price for that colour of stone.

Consequently, the further the stone is away from these ideal values, for just one of the 3 criteria or all 3 at the same time, the more the price will drop.


Our advice:

When chhosing your stone, concentrate primarily on the color you like best, even if you make concessions on the clarity or the weight.



Clarity

By their nature, some substances naturally have a greater degree of clarity. When making your choice you will need to find the natural degree of clarity for the type of stone in question and decide if it has more inclusions thna it should.


We can separate all the stones in 3 classes: * Often without inclusion: Type 1. * Mainly included: Type 2. * Almost always included: type 3. THe stones can be divided into 3 main classes :
  • Type 1 : mostly without inclusions.


  • Type 2 : generally with inclusions.


  • Type 3 : almost always with inclusions





Conclusion :

The value of a stone varies its ranking relative to its type: a clear emerald has a higher price because of the rarity of this stone being so clear (type 3 in the ranking).


When choosing your stone :

  • Avoid stones with inclusions to be in the area of mounting the stone, these inclusions often present a risk of breakage.


  • A stone with no inclusions at all is often a stone or a synthetic imitation.


  • It is also the shape and nature of inclusions that give a stone its esthetic value.



The cut: an often-neglected criterion.



    a. The proportions of a stone:

    By proportion we mean the beauty of the shape a stone, that is to say, its external shape and the ease with which the stone may be mounted.

    1. External contour

        Symmetry of the shape :

           Is the circle perfectly round?
           Is the oval slightly distorted ?, etc.




       Length/width relationship:








    Our advice: avoid unsightly shapes!



    2. The relationship between the thickness of the crown and pavilion depth :

    A 1:3 ratio is better, e.g. 1 mm for the crown (top) and 3mm for the pavilion (bottom).


    Our advice:
    Always be very careful not to have a shallow depth of pavilion: indeed a pavilion angle of less than 42 degrees will (in general) prevent the light from reflecting back and sparkling. An angle between 65 and 42 degrees is acceptable.

    Very deep pavilion: the risk of having a very tall jewel and therefore the weight of the stone is too high relative to its size. Don't forget that a stone is bought by weight.


    3. The table :

    The width of the table iscompared to the width of the stone.

    A ratio of 35% to 65% is within acceptable limits.

    Ratio = (width of the table / width of the stone) x 100 = percentage


    Example :
      Table width = 5mm ;
      Stone width = 10mm
      Ratio = (5mm / 10mm) x 100 = 50%.



    Our advice:

    The table is the window of your stone and you pay particluar attention to this feature.

    4. Misalignment of the pavilion :




    One of the most difficult parts of the stone-cutter's trade is controlling the position of the coloured reflections in the pavilion of the stone..

    Especially with gemstones that have :

    • zones of strong colour.


      • different colours or shades of colour depending on the axis in which we look at the stone (pleochroism). This is the case for corundum (sapphire, ruby, sapphire color), Quartz (citrine, amethyst, smoky quartz, tourmaline ...), and many others.


    These stones often have a misalignment of the point of the pavilion, or even the entire pavilion.

    The skill of the diamond cutter resides in :
    • enhancing a particular area of the stone that is more colourful and therefore more "commercially interesting", distributing the color more evenly throughout the stone (reflection / refraction / pleochroism).


    • removing inclusions that are too visible.


    • and sometimes optimizing the shape of the stone so as to lose the least weight during cutting.



    Our advice:

    A stone with a beautiful colour but no life is infinitely less attratcive than a stone that my have a less beautiful colour but sparkles and catches the eye .
    The misalignment of the pavilion has a direct impact on the shine of your stone.

    A skillful diamond cutter will give your stone a harmonious balance of the criteria listed above.


The weight

This factor is purely commercial.
The stones are sold by the carat (1 carat = 0.2 grams).

The bigger the stone, the greater the value per carat.

For example: a 2.00-carat sapphire is worth less per carat than 3.00-carat sapphire.


From a certain weight, stones of the finest quality are scarce and therefore the price increases exponentially.

Thus, a beautiful 1.5-carat ruby can cost 1,000 Euros per carat, while a 6-carat untreated Burmese one can easily exceed 15,000 Euros per carat, if not 150,000 Euros.



To round off this introduction to choosing a stone, our best advice is:

Take your time, trust your taste, feel free to request a certificate from an independent laboratory, and most importantly, compare a number of stones.



We are at your disposal for any questions and explanations.

Buying coloured stones