The colour.
A fine gem’s colour is one of its most important elements, but it is also one of the most subjective factors. Coloured gemstones are highly diverse, and so too are the moods they can evoke. Orange stands for optimism, yellow for cheerfulness, green for generosity, red for love, and blue for fidelity. The more unusual a colour is, the more ardently people strive to find words to describe its beauty. Imagine a tourmaline that is as blue as a swimming pool, a topaz with a golden gleam that rivals the sun, or an emerald with the verdant hues of a lush garden. Intoxicating as these colours may be, they in fact result from a sober natural process and the gem’s chemical composition. Metals and their compounds are the principal sources of colour in the mineral kingdom. Iron, for example, imbues sapphires with incomparable blueness and chrome gives rubies their redness. The slightest nuances of colour can decisively influence a gem’s value. Furthermore, rare and especially coveted variations exist for nearly every type of gemstone – variations such as the red rhodolite (a special type of garnet) or the pink rubellite, which belongs to the tourmaline family.
The transparency.
Careful scrutiny can uncover tiny inclusions of minerals or foreign crystals in nearly every coloured gemstone. Sometimes known as “Mother Nature’s fingerprints”, such traces of growth are infallible signs of the authenticity and naturalness of a coloured gemstone. As a general rule, the more transparent a gemstone is, the more valuable it is since a gem’s vitality and luminosity result from its transparency.
The rarity and the provenance.
That which is rare, is avidly sought. A good example of this is the red ruby, which ranks among the world’s most precious stones. Alongside rarity, the country of origin is another factor determining a stone’s value since colour varies with the the gem’s location and specific constituents. The best emeralds, for example, featuring an incomparably intense and slightly bluish green colour, are often found only in Colombia. The rarest imperial topaz, whose honey-coloured hue is a real highlight, usually comes from the region around Minas Gerais in Brazil, which is justly famed for its choice topazes.
The weight and the cut.
The carat has served as the unit of measure for the weight of precious stones since classical antiquity. One carat is equal to 0.2 grams, which is the exact weight of a single carob seed, a so called qirat. Each qirat is identical in size and weight, which led them to be used as reliable units of measure. A gemstone-cutter decides on a particular cut depending upon the weight, composition, and colour of the unprocessed stone. Only a perfect cut can fully reveal a gem’s inherent brilliance and liveliness. Unlike diamonds, whose cuts are based on mathematical calculations, coloured stones are cut on the basis of experience and instinct. A smooth cabochon cut is usually the best choice for a stone that is not entirely transparent, such as the red tourmaline found in the photo at right. A large number of facets can help produce diverse refractions of light and a sparkling firework of colours in more transparent gems.
The hardness.
The Mohs scale of hardness, developed more than 150 years ago by Viennese mineralogist Friedrich Mohs to measure the hardness of minerals, is still in use today. Mohs arranged minerals on a scale of 1 to 10 according to their hardness. The diamond, a jewel of incomparable beauty, ranks at the top of Mohs’s scale with a hardness of 10. Generally, a harder stone can scratch a softer one, but stones of equal hardness cannot scratch each other. Corundum (e.g., sapphire and ruby) has a hardness of 9 on Mohs’s scale. The lower end of the gemstone scale is represented by a hardness of 6 where moonstones, for example, can be found. Softer stones gradually lose their gleam because they have no resistance to the abrasive effects of miniscule particles of quartz (7 on the scale) circulating in the air and settling onto polishing cloths. Hardness is also an important factor for gemstone cutters, who must have many years of experience under their belts in order to properly appraise a stone, especially since several crystalline surfaces on one and the same gem can differ in hardness.
