A gemstone is commonly called a gem and known by various other terms such as semi-precious or precious stone, jewel or fine gem. Pieces of raw mineral crystal are cut and polished into gemstones which are used in different forms of jewelry as well as other types of adornments such as bracelets, brooches, watches, et cetera. Besides minerals, some rocks and certain organic materials also serve as crystals used in jewelry and so can be considered as gemstones too. Lapis Lazuli and amber or jet is commonly used rock and organic materials respectively in jewelry. Gemstones by nature are hard, but certain soft minerals are also used in jewelry due to their luster or aesthetic value.
Many gemstones are famous for a number of reasons, among these being their size or beauty, or made famous by the celebrities or famous personalities who wore them or owned them. Listed below alphabetically are some famous gemstones and their striking features:
Alexandrite is a type of Chrysoberyl, and is unique in that it undergoes a change in color depending on the surrounding lighting. Commonly known as the Alexandrite effect, the gemstone undergoes a change from a greenish hue to a reddish one, depending on the source of lighting.
Aquamarines belong to the class of Beryl gemstones and are of the blue or cyan variety. The blue color of aquamarine is due to the presence of traces of iron. Most areas that yield ordinary Beryl are, likewise, rich in yields of aquamarine.
Diamond is a form of hardened carbon that is kinetically very stable once formed and hence referred to as being metastable. Here the arrangement of the carbon atoms forms the diamond lattice which is a modification of the usual face-centered cubic structure that occurs in most crystals.
Opal is a powdery form of silica that is hydrated, containing water of crystallization. The water content in opal, by weight, may be anything between 3 to 21%, but is generally found to be in the range of 6 to 10%. Opals are generally classified as mineraloids due to their amorphous nature.
A pearl is formed over a period of time by living shelled mollusks. It is formed in the soft tissue parts of the clam, in particular the mantle of the shell. Similar to the shell, a pearl is also hard and is made of calcium carbonate. This calcium carbonate is deposited as fine crystals in concentric layers to give a pearl a round or ovoid appearance.
A ruby is a precious gemstone, occurring in the mineral form as corundum, which is a type of aluminium oxide. These gemstones range from pink to blood-red in color. The red tone is mainly ascribable to the chromium content in the mineral, with pink colored gemstones having the lowest chromium and red ones having the highest. Rubies get their name from the Latin word ‘ruber’ meaning red.
Among all the known gemstones, sapphire is the most desired, valuable and precious blue gemstone. Popular by choice, some of its qualities include its luster, hardness, durability and beautiful hue.
Spinel also has the same shades of blue and red colors that are found in sapphires and rubies. They are formed under similar geological conditions and are found in the same rock units and gravel content as sapphires and rubies.
Topaz is a gemstone known since antiquity. Its name is derived from the Greek word ‘topazion’ or the Sanskrit word ‘tapas’ meaning fire, probably referring to the brilliant sheen of the gemstone material.
Classification of gemstones
Gemstones are classified into different types based on chemical composition, optical nature, crystal structure, specific gravity and durability.
On the basis of chemical composition, gemstones may be composed of a single chemical element such as diamond which is made up of carbon, or a chemical compound that has a fairly simple composition such as quartz which is a form of silicon dioxide (SiO2) or as a complex mixture of many compounds and elements such as the gemstones belonging to the garnet family. Garnets contain a mix of magnesium, iron, aluminum and calcium silicates in varying proportions. Most of the known gemstones are obtained from mineral crystals that are either oxides or silicates (compounds of oxygen and probably silicon) formed over eons as the earth’s crust cooled.
Based on crystalline structure, gemstones may be categorized as discrete or microscopic crystals or amorphous (non-crystalline) material. Discrete crystals may be built up of a single crystal or many, diamond being an instance of such crystals. Microscopic crystals on the other hand, are made of large collections of minute crystals (crypto-crystalline) as in chalcedony. Amorphous gemstones are composed of powdery, non-crystalline material, a common example of this category being opal. By rule of nature, rapid cooling of molten rocks gave rise to smaller crystals, while slow cooling resulted in the larger crystals. Besides the nature of the crystals, gemstones can also be categorized on the basis of the symmetry of the formed crystals. Besides the amorphous sources of gemstones, organic sources of gem material that are non-crystalline in nature include shells and bones.
Stones get their optical features based on their crystalline structure and chemical makeup. Color, optic character, refractive index, dispersion, fluorescence and phosphorescence are some visual characteristics that influence the nature and quality of stones.
Gemstone materials have wide ranging densities. Amber which floats in salt water has a density close to that of water, while Hematite has a density greater than five times that of water. In practice, geologists use specific gravity or relative density, which is the ratio of the density of the gemstone in relation to that of water when comparing gemstones. Due to this very reason, we can have two gemstones of similar sizes but having different weights or having similar dimensions, but different weights, for example, a brilliant diamond of one carat and a brilliant ruby that is 1.55 carats, but both with similar dimensions and size.
Durability of gemstones is measured in terms of two qualities, hardness and toughness. Though often misunderstood and used interchangeably, these qualities refer to two characteristics. Hardness refers to resistance to scratching or piercing, whereas toughness implies resistance to breakage. The durability of a gemstone is thus a combination of these two measures.
Some terms used in characterizing gems
Gemstones may be characterized based on a number of qualities or features or properties. Among these criteria are refractive index, flaw, and water of the gemstone. The refractive index will also be influenced by the specific gravity, hardness, fracture and dispersion of the material of the gemstones. Based on their refractive property, gems and minerals may be luminescent, form distinct and unique refraction spectrums, display pleochroism or cause double refraction. Gemstones or crystal minerals may have some extraneous material or crystal abnormality as an inclusion within the quartz structure. Such anomalies are referred to as crystal or material flaws. Based on the water of the gemstone, gems can have varying luster, transparency and brilliance. Grading gemstones based on the water in the gemstone is also a usual pattern. Clear transparent gems would get a first water rating, while gems with reduced transparencies would be graded second or third water.
Gemstones are available in a wide variety of colors. Sometimes, the gemstones having the same material and chemical composition exhibit different colors. The colors of each gemstone depend on its atomic structure. Even in the same type of gemstone, different colors are available, for example, Sapphire is available in blue, orange, pink and many other colors. It is because the atoms keep on replacing one another. This process is undergone by a very few atoms only; may be one out of millions of atoms. But that single atom is sufficient to change the color by absorbing certain colors of the daylight and leaving the others. This atom is termed as impurities in the minerals. The Beryl mineral is colorless in its original form, but turns into emerald when combined with impurities of chromium.
Gemstones may not have their final sheen or luster until they are subjected to some processes to improve their color and clarity. Based on the process and the extent of the process, the value of the gemstone can be affected. Certain treatments result in a more stable gem, and are widely accepted and practiced. There are, however, some other treatments that are not commonly accepted, but used on rare occasions as the color resulting from such treatments is unstable and the gemstone may revert to its original shade. Some common processes used to improve the luster and colors of a gemstone are given below:
Heating a gemstone to high temperatures, high temperatures under pressure or high temperatures in oil can improve its clarity or color. Some common gemstones that are heated to improve their color or clarity include amber, aquamarine, ruby and even diamond heated under pressure to make it colorless.
Radiation is a common process used to impart the trademark blue hue to almost all forms of topaz. They are changed from a white shade to lighter as well as darker shades of blue as in ‘London’ blue topaz through the irradiation process. Virtually all blue topaz, both the lighter and the darker blue shades such as "London" blue, has been irradiated to change the color from white to blue. The majority of green quartz type of gemstones (Oro Verde) is also subjected to radiation to impart to it its characteristic yellow-green color.
Emerald and turquoise may contain some natural fissures within their crystal structure. These fissures are filled with colored wax or oil to disguise them also give the gemstones a better appearance and refined clarity.
Fracture filling is another common treatment used to improve the appearance of gemstones like emeralds, sapphires and diamonds. This and similar treatments, however, are pretty easy to detect. In 2006, lead glass was used to fill large fissures in rubies that were over 10 carats to improve their appearance. This was, however, detected and the magic trick with ‘glass filled rubies’ received wide publicity.