Gemstones: (first row) turquoise, hematite, chrysocolla, and tiger's eye; (second row) quartz, tourmaline, carnelian, pyrite, and sugilite; (third row) malachite, rose quartz, snowflake obsidian, ruby, and moss agate; (fourth row) jasper, amethyst, blue lace agate, and lapis lazuli

We often hear and speak about jewels, gems or gemstones. Most of us also know that these are expensive, and, we associate them with jewelry.

What are gems? The simplest explanation is, most of them are minerals, and some are rocks or pebbles, and pearls (CaCO₃ or calcium carbonate in minute crystalline form) naturally grow within living shelled mollusks.

One of the most precious and desired gems, diamonds are pure carbon (C) the same chemical substance in graphite, and the major component of coal. Similarly, rubies are made of aluminium oxide (Al₂O₃).

Grading and evaluation of gemstones, from the times of the Ancient Greeks, are done by judging them with the help of naked human eyes (with 20/20 vision), because there is no universally acceptable grading system. But since the 1950s, the Gemological Institute of America (GIA) introduced a grading system for diamonds with 10x magnification as the standard for grading clarity.

Chemical composition is an important factor to determine the quality of gemstones, and gemologists apply chemical tests first to evaluate them. Also, chemical composition is responsible for the color of gemstones. For instance, ruby is the red variety of corundum (crystalline aluminium oxide (Al₂O₃) with traces of chromium, iron and titanium), which in any other color is sapphire. Even sapphires are available in various shades of blue, yellow, pink and orange-pink colors.

The mineral beryllium aluminium cyclosilicate, with the chemical formula Be₃Al₂(SiO₃)₆, commonly known as beryl, which is colorless, is the same chemically as the gemstone emerald which acquires green color because of the atoms of chromium (Cr) getting mixed with it and making it impure. The same beryl becomes the pink-colored morganite with manganese (Mn), blue-colored aquamarine with iron (Fe), and the gemstones red beryl, goshenite (colorless) and heliodor (yellow) are made up of beryl with other elements or compounds as impurities.

After the exact chemical composition of gemstones was made available by scientists, there have been efforts to create gems in laboratories. And the results have been highly encouraging to manufacture synthetic and artificial gemstones on a commercial scale. One such artificial gem is the cubic zirconia, a synthetic diamond composed of zirconium oxide, and another is moissanite, followed by sapphires, rubies and emeralds created in labs, costing only a fraction of the natural gemstones. Now bulk-manufactured small synthetic diamonds are used as industrial abrasives, and larger synthetic diamonds are available in high gem-quality and multiple carats.

Several minerals like salts and even gems like diamonds exist in nature as crystals. Crystals can have very complex to the simplest geometrical shapes and structures. So, it is natural that many gems exist as crystals. Hence gemologists classify gemstones based on their crystal systems such as monoclinic, trigonal, cubic, etc. Diamonds have a cubic crystal formation and are often found as octahedrons.

Based on crystal formation, crystalline gems are polished, shaped or cut. Transparent gemstones are normally faceted as diamonds. Opaque gems like variscite, turquoise, opal, and similar ones are usually cut as cabochons.

What makes gems so precious and attractive? One of the main factors is their color. And what is color?

The color given out by any object is because of the nature and composition of light, or daylight, which is transparent. Light is a form of electromagnetic radiation that is visible to the human eyes. The visible range of light is in the frequency range of 405 THz to790 THz. Infrared rays (lower than 405 THz) and ultraviolet or UV (above 790 THz) are not visible to the human eyes. When rays of light travel through a material (must be transparent or translucent), some frequencies of the light are absorbed by it. The unabsorbed part reaches the eyes. For this reason the gemstone ruby appears red, because it absorbs all frequencies of light other than red.

Radiation also plays an important part in making jewels colorful. For example, blue topaz, all the lighter and the darker blue shades, is irradiated to change the color from white to blue. Similarly, the green quartz (Oro Verde) is also irradiated to impart them the yellow-green color.

The effect of heat can make dramatic changes to many gemstones in terms of their color or clarity. Most of the citrine is made by heating amethyst. When heat is applied partially, it becomes ametrine, a combination of amethyst and citrine. Similarly, the gem aquamarine is heated to get rid of the yellow coloring, or to transform the green color to pure blue. The tanzanite gems are heated at low temperatures to change the brown tones and to give them blue or purple color.

Identifying gems, properly grading them and evaluation are quite often not a layman’s job. Gemologists look for properties such as the refractive index, specific gravity, luster, cleavage, light dispersion properties, hardness and fracture as some of the basic characteristics of precious and semiprecious stones. Also, there are some rare, unusual and infrequently available gemstones. Only, gemstones connoisseurs can understand and evaluate them, as in the case of axinite, cassiterite, andalusite, clinohumite and red beryl.

All the above facts may make it look somewhat difficult for gem and jewelry consumers to evaluate and buy their favorite gemstones or ornaments in which they are embedded on. Not quite so! There is a simple system called ‘Four Cs’, which stand for ‘color, cut, clarity and carats’ that has been introduced to guide diamond consumers.

Moreover, now most people do not depend on their neighborhood jeweler, or village jeweler for their purchase of gems and jewelry. Instead they go to the most reputed brands and high-end shopping arcades. So, you will be buying from a reputed brand or the jewelry merchants popular with your family or social circles. So, you simply select and buy jewelry based on the brand name of the jewelry designer or the current fashion trends so that you don’t even need the ‘Four Cs’.

Sometimes, there arises confusion as to what are precious stones, and what are semiprecious stones. Traditionally, diamonds, opals, rubies, sapphires, emeralds, amethysts and pearls (though pearl is not a gemstone in the true sense) have been classified precious stones. But the amethyst seems to have fallen out of grace since the 19th century, when it was discovered in Brazil from where it became available in bulk and cheap. Until the last century stones like aquamarine, peridot and cat’s eye (cymophane, some people call it tiger’s eye also) have been graded as precious stones.