GemSelect Newsletter - The Science of Gemstone Classification
Gemstones by Type
The Science of Gemstone Classification
Have you ever wondered how gemstones are classified? Well, according to the science of mineralogy, gemstone classification begins by distinguishing various gemstone groups based on their crystal structures and associated chemical composition. Gemstone groups are then subdivided into separate types and lastly, varieties. The process seems simple enough, but in actuality, most average consumers are unaware that groups, types and varieties are actually different gem classification levels. The science of gemstone classification was defined by former GIA graduates, Cornelius Hurlbut and Robert Kammerling, and remarkably, their system is still used by gemologists today.
When two or more gem types have a similar chemical composition, crystal structure or physical qualities, they are defined as a group. There are 16 gemstone mineral groups, consisting of beryl, chrysoberyl, corundum, diamond, feldspar, garnet, jade, lapis lazuli, opal, peridot, quartz, spinel, topaz, tourmaline, turquoise and zircon. Interestingly, there are several gem "groups" that are actually stand-alone gem types, such as tourmaline, zircon, topaz and spinel. However, each of these types has multiple variations available, therefore justifying group classification status.
Gemstone types have varying chemical compositions, as well as crystal structures. There are approximately 130 or more different gemstone types on the market today, with new discoveries being added. Chemical compositions can range widely, from complex mixes of various compounds to simple, single chemical elements, like diamond which is composed of only carbon. Although diamond may have a simple composition, its crystal structure is quite complex. Crystal structures can also vary from simple single structures to immensely complicated clusters of microscopic crystal. Some types may even possess a non-crystalline structure such as opal, referred to as an amorphous structure. Inorganic gems are classified based on chemical composition, as well as crystalline structure similarities, whereas organic types are classified by chemical composition only. Examples of organic species include pearl, coral, amber and ivory.
Gem varieties are subdivided and branched from gem types. A perfect example would be corundum, which is a group. Blue corundum is known as sapphire and red corundum is ruby, both of which are gemstone types. Sapphires that possess asterism become a variety of sapphire - star sapphire. Varietal classifications are based on optical qualities including color, optical phenomena, color distribution and transparency.
Color is produced through the absorption and transmission of light at certain frequencies. The wavelengths of specific electromagnetic vibration determine the visible color perceived by the human eye. Colors are described using a combination of varied hues, tones and intensity levels. Some examples of gem varieties classified by color include citrine, the yellow-gold variety of quartz; and amethyst, which is the violet variety of quartz.
Optical Phenomena: The most common of optical phenomena is iridescence. Iridescence includes traits such as orient, play of color and labradorescence. Fire agate is an example of a gem variety classified by optical phenomena. Fire agate is simply brown agate with iridescence, caused by layers of plate-like crystals of iron oxide, or limonite. As a result of the reflection of light from thin layers of limonite, red, gold, green and blue-violet colors are displayed. Some gem varieties are distinguished by chatoyancy (the cat's eye effect), such as cat's eye tourmaline or cat's eye quartz. Other types of optical phenomena include adularescence, aventurescence, asterism (the star effect) and color change.
Color Distribution: Color distribution refers to the distribution of solid colors and patterns on a gemstone. Some gemstones distribute colors and patterns so unique that a separate varietal name is needed in order to properly classify them. One example is onyx, which is essentially agate formed with parallel rather than curved band patterns.
In the gem trade, transparency refers to a gem's ability to transmit light, and gems are often classified by their transparency. Gemstones can be transparent, translucent or opaque. Colorless quartz is an example of a transparent variety, whereas agate and moonstone are typically translucent. Both jasper and tiger's eye are considered to be opaque, because no light is passed through these varieties.
Organics & Synthetics
Organic gems are naturally occurring gems that are the result of biological processes. This is a small group of gems including jet (bituminous coal), coral, ivory, pearl and amber.
Synthetic gems were created in a laboratory setting. Synthetic gems have "natural" composition, but they were not naturally occurring. When compared to their naturally occurring counterparts, synthetic gemstones have identical chemical composition and crystal structure, including specific gravity and other various optical qualities. It should be noted however, that not all lab-created gems are considered to be synthetic. This is because some lab-grown gems utilize unnatural ingredients, such as lab-grown opal, which is composed of silica (70-80%) and bonding agents (20-30%).
Synthetics are very often confused with "simulated" gems, but simulated gems are simply imitations possessing only optical similarities. A diamond simulant known as cubic zirconia is a perfect example of a simulated gem; CZ appears to be the same as natural diamond, but the chemical and crystal structures are worlds apart.
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