Gemstone Chemistry and Mineral Composition Explained

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.