A Guide to Gem Classification
For centuries, gemstones have been admired for their beauty, rarity, and symbolism, but behind their appeal lies a classification system.
Gemmologists use scientific methods to categorise gemstones, based on various characteristics such as origin, structure, and physical properties. Whether you’re a casual collector or a professional gemmologist, understanding the ways in which gems are classified is essential to appreciating their true value.
Here's a comprehensive guide to how gems are categorised:
Natural vs. Synthetic Gems
Natural gemstones are formed through geological processes over millions of years, deep within the Earth. These include familiar stones like diamonds, sapphires, and emeralds. Natural gems often carry the attraction of rarity and the mystique of ancient origins.
In contrast, synthetic gems are created in controlled laboratory environments, mimicking the conditions under which natural gemstones form. Although man-made, synthetic gems often have an almost identical chemical composition and physical properties as their natural counterparts. Synthetic diamonds, rubies, and other gemstones are virtually identical to their natural versions and are gaining popularity for their affordability and ethical sourcing.
Natural rough diamond
Synthetic corundum boules
Precious vs. Semi-Precious Stones
Historically, gemstones were classified as either "precious" or "semi-precious" based on their perceived value. Precious stones included diamonds, rubies, emeralds, and sapphires, while all other gems were labelled as semi-precious.
This distinction is now considered outdated because the market value of gemstones depends largely on their rarity, quality, and demand, rather than this traditional classification. For example, amethyst was once a precious stone before larger deposits were found, decreasing its rarity. Today, the term "semi-precious" can be misleading, as many so-called semi-precious stones like spinel or tourmaline can be highly valuable, depending on their attributes.
Crystalline vs. Amorphous Gems
A key distinction in gemstone classification lies in whether the stone is crystalline or amorphous. Crystalline gems have an orderly, repeating atomic structure. Common examples include quartz, beryl, and garnet. These stones tend to form beautiful, angular shapes and exhibit the optical phenomenon of birefringence, in which light travels at different speeds through different axes of the crystal.
Amorphous gemstones, on the other hand, lack this organised atomic structure. Opal and amber are prime examples. Amorphous gems often have a more fluid appearance and display unique optical properties, like the iridescence of opal, which results from the stone's internal play of light rather than its atomic structure.
Rough opal
Organic vs. Inorganic Gems
Organic gems are derived from biological processes. Pearls, for example, form in molluscs, while amber originates from tree resin that has fossilised over millions of years. Coral, another organic gemstone, is made of calcium carbonate secreted by marine organisms.
Inorganic gems, on the other hand, are mineral-based and form through natural geological processes. Most gemstones fall into this category, including diamonds, sapphires, and tourmalines. Inorganic gems are classified based on their mineral species, crystallographic properties, and composition.
Colour, Clarity, and Cut
Gem classification also considers aesthetic qualities like colour, clarity, and cut. Colour is often the most important attribute, as it directly affects a gemstone’s beauty and desirability. Pure gemstones, like diamond, are typically colourless in their purest state, with impurities giving rise to vibrant hues such as the deep reds of ruby or the lush greens of emerald.
Clarity refers to the absence of inclusions or internal imperfections, which can detract from a gem’s appearance and value. Some gemstones, like emeralds, are known for naturally having inclusions, while diamonds are often valued for their exceptional clarity.
Cut refers to the craftsmanship behind shaping the gem, impacting how light interacts with it. A well-cut stone can enhance its natural colour and brilliance, while a poor cut can diminish its visual appeal.
Faceted aquamarine and topaz gemstones
Aggregates vs. Single Crystals
Certain gems, like jade and chalcedony, are classified as aggregates. These stones consist of groups of microscopic crystals forming a single unit, giving the gem a smooth, often waxy appearance. Aggregates tend to have a more subdued lustre compared to single crystals but can still be quite beautiful.
Single-crystal gemstones, like sapphire or aquamarine, consist of a single, continuous crystal structure. These gems often display sharper facets and reflect light more dramatically, contributing to their higher value in many cases.
Natural citrine crystals
Close up of agate which is a variety of chalcedony (polycrystalline quartz)
Mineral Species and Varieties
Minerals are classified into species based on their chemical composition and crystal structure. For example, all varieties of quartz, including amethyst, citrine, and smoky quartz, belong to the quartz species due to their common chemical makeup (silicon dioxide).
Within each species, there are gemstone varieties distinguished by colour, inclusions, or other factors. For example, the mineral corundum can form either ruby (if red) or sapphire (if any other colour). Understanding the species and varieties of gemstones helps gemmologists assess their quality, rarity, and value.
Conclusion
Gemstone classification is a nuanced process, incorporating both scientific principles and aesthetic judgement. Whether distinguishing between natural and synthetic stones, assessing colour and clarity, or identifying a gem’s mineral species, understanding these factors allows collectors, gem enthusiasts, and professionals alike to appreciate the intricate beauty and value each gemstone holds. By learning how gems are classified, you not only gain insight into their origins and qualities but also develop a deeper connection to these natural treasures.
