Focus : What is Gold ?

Known, loved, sought-after, coveted, amassed, envied, disputed and used by many civilizations since mankind knew how to master the secrets of metals, gold has always been one of the most precious and symbolic materials. However, more than its color and its brilliance, powerful and almost irresistible fascination, the unique properties of gold made it the favorite metal to adorn Gods.

Auri sacra fames, so said Virgil (The Aeneid, III, 57, 29-19 BCE): this sacred, intense, immoderate hunger for the precious metal has prompted humanity to discover new worlds, to conquer the planet. But beyond the symbols, myths and legends associated with gold, do we really know this yellow and shiny metal? What type of atom does it contain? How is its inner structure organized and which unique properties derive from it? How does nature produce, shape, and make gold available to mankind? How does mankind exploit it, transform it, master it and prepare it for the crafting of extraordinary objects?

Chemical Aspects of Gold

Gold is the chemical element with the atomatic number 79 in the Mendeleev’s Periodic Table of the Elements with the symbol Au (from the Latin word aurum). This means that the nucleus of the gold atom contains 79 protons. For the chemist, to understand matter and its properties is to know the structure of the atoms that compose it. These 79 protons, 118 neutrons, and 79 electrons are the building blocks of the gold atom that determine how they organize themselves to form matter. They also explain how they will interact with other simple (oxygen, mercury, etc.) or complex elements (water, acids, etc.) and thereby, characterizing the properties of matter.

The most significant properties of gold, which made it a material of choice for jewelry-making, are its high malleability and exceptional ductility. Malleability is the ability to be pressed into shape without breaking (by rolling, beating, etc. For example, to produce 0.0001 mm extremely thin gold leaves). The hardness (resistance to scratch) of the metal is therefore low, measuring 2.5 out of 10 on the Mohs scale (from 1 for Talc to 10 for Diamond). 

As regards ductility, it is the ability to resist plastic deformation (stretching for example) without breaking. The ductility of gold makes it possible to obtain from 1 gram of material a wire measuring 0.00001 mm in diameter and over 2 000 meters in length! 

The atomic organization of matter also influences the way light interacts with it. Gold possesses a high metallic luster. In other words, it reflects almost all the light received (95% to be exact). This optical property, coupled with its mechanical properties, make gold a material of choice for many industries, such as the aerospace industry for example.

Mineralogical aspects of Gold

As part of the cubic system, gold can be found in nature as octahedral, dodecahedral, and cubic crystals, as well as all combinations and variations of these forms (ill. 1). However, these characteristics are extremely rare because they are often deformed by different geological movements. More commonly, they are found in the form of nugget.

Figure 1 : This extremely rare gold crystal shows octahedral shapes typical of the cubic system in which it crystallizes. Mineral Museum Collection – MIM – Beyrouth, Liban

As with many minerals, we can identify a primary deposit if the crystals are found in situ, o the site where they were formed, or directly transported from the depths (ill. 2). The secondary deposits, on the other hand, are places where crystals moved by erosion are found, often in mass.

Figure 2 : Specimens of rough gold as they are found in primary deposits Mineral Museum Collection – MIM – Beyrouth, Liban

Primary gold deposits are often contained in quartz veins. Gold can develop into crystals that are sometimes several centimeters in size. These veins, however, often contain only a very low concentration of gold.

In the case of secondary deposits, accumulation of fine gold dust or small grains, among other heavy minerals, are formed into nuggets that settle as placers in streams or rivers. This type of gold is sometimes referred to as “river gold” as opposed to “mountain gold” from primary deposits.

Jewelry-Making Aspects of Gold

Pure gold is rarely used in modern jewelry-making because it is too soft to withstand everyday wear or to set gems. It is therefore mixed with other metals to obtain alloys of gold, also referred to, rather confusedly, as “gold”. In order to determine the purity of gold alloyed with other metals, the concept of “karat” is used. This karat should not be confused with the metric carat, which is a unit of mass equivalent to 0.02 gram. Pure gold is called “24-karat gold” (100% gold). On this basis, the percentage of gold content in the alloy is calculated. An alloy containing 91.6% gold will be called “22-karat gold”, and hence 18-karat (58.5%) or 9-karat gold (37.5%), even if the latter contains only a small amount of pure gold. The reason why the proportion of pure gold is measured out of 24 parts comes from the “gold penny” or “solidus” minted by the emperor Constantine I around 310 CE to replace the “aurei” of Caesar (the first to mint a gold coin). Weighing 4.5 g (or 24 old carat), the “solidus” was composed of pure gold. It was the standard of the Byzantine monetary system for more than ten centuries, as a currency all around the Mediterranean countries. It is referred to as a reference for financial value but also a standard for both mass and purity of gold, two concepts now defined by the same term carat/karat.

In addition to making the gold alloy harder and more resistant, mixing different metals will also change the original yellow color of pure gold. The “yellow gold” alloy contains silver and copper, “white gold” mostly contains silver and zinc, “rose gold” mostly copper with a little silver. There are also red or green golds, which are rarer, depending on the input of other metals. The percentage of each metal for each alloy is more or less standardized, but each jeweler has its own standards and preferences. The diversity of alloys in the market makes it possible to satisfy each designer and adapt to each situation. Gold is a malleable, multiform and even multicolored metal that keeps fascinating Mankind, always and for evermore!