The Big Four

Guide to Precious Stones

Rubies, Sapphires, Emeralds and Diamonds.
These gemstones are commonly referred to in the jewelry industry as “The Big Four“, the four precious stones

These gemstones are evaluated and classified in accordance with the 3 following criteria:

    • Clarity : The transparency of a stone is affected by the amount and size of foreign bodies, called inclusions, in its crystal structure.
    • Color : the degree of intensity, neither too bright nor too dark (except for colorless diamonds).
    • Brilliance : The appearance of the stone when it reflects rays of light. It depends mainly on the cut of the stone. The better the cut, the better the ability of the stone to reflect light.

  • The rarity of a gemstone is determined by its availability on the market.  The scarcer the gemstone, the most expensive it will be.

  • Essentially based on three properties: hardness, toughness and the resistance to chemical agents.

    The hardness of a mineral reflects the resistance of the crystal structure to mechanical stress. Essentially, it is how well a material resists scratching.

    The hardness measurement is empirical, and the Mohs scale of mineral hardness is composed of 10 mineral hardness reference points. 

    When it comes to evaluating a gem value, and especially a colored precious stone, three other elements must be taken into consideration :

  • The geographical origin of a colored precious stone is never a guarantee of quality but can increase its value (gem from a depleted mine or displaying a typical region’s color…).

  • Measured in carats (1 carat = 0.20 gram)

    The more the carat weight, the rarer the stone is. But bigger is not necessarily better (carats saved/retained at the expense of a good cut, uneven color, more and/or larger inclusions…)

  • The demand for fine gems is significantly higher than the supply. For that reason, the vast majority of precious stones are artificially enhanced. Several methods are available, including:

    • Irradiation: The exposure of a diamond to radiation for enhancing its color.
    • Heat treatment : The exposure of a gem to high temperatures for enhancing  its color and/or clarity.
    • Oil or resin treatment (also called “filling”) : Inclusions in emerald are most of the time filled with oils, waxes, and artificial resins to reduce the visibility of the fractures and improve the apparent clarity.
    Side note : Synthetic stones have been available since the late 19th century. They have the same optical and chemical characteristics as their natural counterparts but are grown in a laboratory. Slight different characteristics between the two, such as natural inclusions within the gems, allow gemologists to distinguish a mineral origin from an artificial one.