How Are Gemstones Classified and What Makes Them Unique?
A gemstone is also called a fine gem, gem, jewel, semi-precious stone, or precious stone. It is a piece of mineral crystal in the form of cut and polished, which can be used to make jewelry or other adornments. However, rocks like opal and lapis lazuli and the occasionally organic materials, which are not minerals like a jet, pearl, and amber, are also used for jewelry and are thus often considered to be gemstones too. Most gemstones are hard, but due to their luster or other physical properties possessing artistic appeal, a few soft minerals are used in jewelry.
Characteristics of a Gemstone
The traditional classification in the West, which goes back to the ancient Greeks, begins with a distinction between semi-precious and precious; similar distinctions are prepared in other cultures. The precious stones are ruby, diamond, emerald, and sapphire in current use, like the other semi-precious gemstones. This particular distinction reflects the scarcity in ancient times of the respective stones and their quality: in their purest forms, all are translucent with fine color, except for the colorless diamond, and very hard, with the hardnesses ranging from 8 to 10 on the Mohs scale.
Whereas the other stones are classified based on their translucency, color, and hardness. The traditional distinction does not specifically reflect modern values. For instance, while grenets are comparatively inexpensive, a tsavorite, a green grenet, may be much more valuable than a mid-quality emerald. Another unscientific term for the semi-precious gemstones, which are used in archaeology and art history, is hardstone. Usage of the terms 'precious,' 'semi-precious' in a commercial context is arguably misleading, and it deceptively implies certain stones are intrinsically more valuable compared to others, which is not necessarily the case.
Gemstones are identified by gemologists in modern times, who describe the gems, including their characteristics using technical terminology specific to the gemology field. The first and foremost characteristic a gemologist uses in gemstone identification is its chemical composition. For example, rubies of aluminium oxide (Al2O3) and diamonds are made of carbon (C). Several gems are crystals, which are classified by their crystal system, like trigonal or monoclinic, or cubic. Another term that can be used is a habit, and the gem form is usually found in. For example, diamonds, having a cubic crystal system, are often found as octahedrons.
Classification
Gemstones can be classified into multiple species, varieties, and groups. For example, ruby is the red variety of the corundum species, while any other color of corundum can be considered as sapphire. Other examples are aquamarine (blue), emerald (green), goshenite (colorless), red beryl (red), morganite (pink), and heliodor (yellow), which are all varieties of the mineral species beryl.
Gems are characterized by the factors of dispersion, refractive index, hardness, specific gravity, fracture, luster, and cleavage. They can exhibit double refraction or pleochroism. They can have luminescence and a distinctive absorption spectrum.
Flaws or Material Within a Stone Can Be Represented as Inclusions.
Gemstones can also be classified in terms of their "water." This is a recognized grading of the luster of gems, "brilliance" or transparency. Extremely transparent gems are known to be the "first water," while the less transparent gems are the "second" or "third water."
Value
Gemstones contain no universally accepted grading system. Diamonds are graded using a system, which was developed in the early 1950s by the Gemological Institute of America (GIA). All gemstones were graded historically using the naked eye. A key breakthrough was used in the GIA system: the inclusion of 10x magnification regardless of the grading transparency standard. Still, other gemstones are graded using the naked eye.
Grading
There exists a number of laboratories where grade and provide reports on gemstones. A few of them are listed below:
IGI - International Gemological Institute, independent laboratory for the purposes of grading and evaluation of jewelry, colored stones, and diamonds
GIA - Gemological Institute of America, the major provider of diamond grading reports and education services
The Diamond High Council, HRD Antwerp - Hoge Raad Voor Diamant, Belgium is one of the oldest laboratories of Europe, where its primary stakeholder is the Antwerp World Diamond Centre
American Gem Trade Laboratory, which is the part of AGTA - American Gem Trade Association, a jewelers trade organization and colored stones dealers
AGS - American Gemological Society is not as widely recognized nor as old as GIA
AGL - American Gemological Laboratories, which is owned by Christopher P. Smith
Every laboratory has its own methodology in the evaluation of gemstones. One lab names a stone "pink" and the other lab calls it "padparadscha." A lab can conclude a stone is heat-treated, while another lab might conclude that it is untreated.
Cutting and Polishing
A few gemstones can be used as gems in the crystal or in other forms where they are found. However, most of them are cut and polished for jewelry usages. The two major classifications are stones cut as smooth, and the dome-shaped stones are known as cabochons, and stones which are cut with a faceting machine by polishing the small flat windows are known as facets at regular intervals and at exact angles.
FAQs on Gemstone in Chemistry: Meaning, Types & Key Properties
1. What is a gemstone from a chemical standpoint?
From a chemical standpoint, a gemstone is a naturally occurring mineral crystal with a defined chemical composition and a specific, ordered atomic structure. Key characteristics that make it valuable are its beauty, durability, and rarity. For example, both ruby and sapphire are gem varieties of the mineral corundum (Al₂O₃), with their different colours arising from trace metallic impurities.
2. How do impurities and structural defects create colour in gemstones?
The colour in many gemstones is a result of chemical impurities, specifically transition metal ions, within the crystal lattice. This is explained by crystal field theory. For instance, a ruby is red because chromium ions replace some aluminium ions in the corundum structure, absorbing yellow-green light and transmitting red light. Similarly, trace amounts of iron and titanium create the blue in sapphires. Structural defects can also trap electrons, creating 'colour centres' that absorb light and produce colour, as seen in some diamonds.
3. What is the difference between a precious and a semi-precious gemstone?
Historically, the distinction between precious and semi-precious gemstones was based purely on rarity and value. The four traditional precious stones are diamond, ruby, sapphire, and emerald. All other gemstones were considered semi-precious. However, this is an outdated commercial term, as some semi-precious stones like alexandrite or tsavorite garnet can be far more valuable than a low-quality precious stone.
4. What are some common treatments used to enhance gemstones?
Gemstone treatments are processes used to improve a gem's appearance or durability. Common examples include:
- Heating: Used to lighten, darken, or change the colour of stones like sapphire and ruby.
- Irradiation: Used to induce colour in stones like blue topaz and some diamonds.
- Oiling/Fracture Filling: Used to hide surface-reaching fractures and improve the clarity of emeralds.
- Dyeing: Used to add colour to porous gems like agate or low-quality turquoise.
5. Why is the Mohs scale of hardness important for identifying and using gemstones?
The Mohs scale of hardness is crucial because it measures a gemstone's relative resistance to scratching, which indicates its durability for use in jewellery. The scale ranks minerals from 1 (softest, like Talc) to 10 (hardest, like Diamond). A gemstone with a high Mohs rating, like sapphire (9) or diamond (10), is ideal for daily-wear items like rings because it resists scratches from everyday exposure to materials like dust (which contains quartz, hardness 7).
6. What is the chemical difference between a natural and a synthetic gemstone?
Chemically, there is no difference between a natural and a synthetic gemstone. A synthetic, or lab-grown, gemstone has the same chemical composition, crystal structure, and physical properties as its natural counterpart. The only difference is its origin. Natural gems are formed through geological processes over millions of years, while synthetic gems are created in a laboratory under controlled conditions. For example, a lab-grown diamond is pure carbon crystallized in an isometric system, identical to a mined diamond.
7. Beyond jewellery, what are some important industrial applications of gemstones?
Many gemstones have important industrial applications due to their unique physical properties, primarily hardness and optical clarity. Key examples include:
- Diamond: Used in cutting, drilling, and grinding tools due to its extreme hardness (10 on the Mohs scale).
- Sapphire: Used for scratch-resistant watch faces, optical windows, and electronic wafers because of its hardness (9) and transparency.
- Quartz: Used in watches and electronics for its piezoelectric properties, which allow it to maintain a precise frequency.
- Ruby: Used in the creation of lasers (the first laser was a ruby laser) and as bearings in high-end watches.
8. What are the nine Navaratna gemstones recognized in Vedic astrology?
The Navaratna, or nine gems, are a group of gemstones that hold significant cultural and astrological importance. Each stone is associated with a celestial body. The nine gems are:
- Ruby (for the Sun)
- Pearl (for the Moon)
- Red Coral (for Mars)
- Emerald (for Mercury)
- Yellow Sapphire (for Jupiter)
- Diamond (for Venus)
- Blue Sapphire (for Saturn)
- Hessonite (for Rahu, the ascending lunar node)
- Cat's Eye (for Ketu, the descending lunar node)
9. What makes a gemstone rare?
A gemstone's rarity is determined by the specific and unusual geological conditions required for its formation. Factors include the precise combination of temperature, pressure, and chemical elements. For example, Bixbite (Red Beryl) is extremely rare because it requires both beryllium and manganese to be present and crystallise together, a highly improbable geological event. Other rare gemstones include Painite, once the world's rarest mineral, and Hibonite.
