Gem Stone Guide


The term gemstone is applied to any of the one hundred or so naturally occurring minerals (diamond, ruby, sapphire, etc.) and organic materials (amber, pearls, ivory, coral, etc.) that are used for personal adornment or display. However, most of the popular gem materials encountered today are inorganic. The first use of a mineral or organic substance for personal adornment remains the subject of debate. But it is generally accepted that their use dates back at least ten thousand years.

To be classified as a gemstone, an organic or inorganic material must possess beauty, rarity, and durability. A gem’s beauty comes from its brilliance, fire, luster and color. In their rough form these attributes are often masked. Only after cutting and faceting and often other enhancements, can the true beauty be revealed. Durability refers to a combination of the material’s hardness (ability to withstand scratching), toughness (ability to resist breakage), and stability (ability to withstand chemical or physical change resulting from heat, light or chemical exposure).

There are approximately 2,700 different minerals on record. However, less than one hundred are regarded as a gemstone or ornamental variety for jewelry use and fewer than 50 are regarded as common. Gems are grouped by their composition and classed by species and variety. Minerals form at different depths within the Earth. Some form close to the surface when solutions containing minerals and water experience a change (usually the evaporation of water) triggering the crystallization process. To illustrate this process one can dissolve salt into warm water. Let the water stand for a few weeks at room temperature and as it evaporates the salt crystals will form as the amount of water diminishes to a level where the salt can no longer be held in solution. Most gem varieties formed at depth from the elements present in the Earth’s mantle. Initially these elements may be contained in molten mixtures called magma. As the molten material was forced closer to the Earth’s surface by various processes, the mixtures cooled and minerals crystallized. Because silicon, oxygen and quartz are abundant in the Earth, it is no surprise that most gems are silicates, oxides and quartzes.


Gemstones have certain properties that gemologists rely on to aid in identification. These include density and refractive index and visual characteristics among others.


Density refers to the physical property of matter that describes how tightly packed together the atoms of an element or molecule of a compound are. The more tightly packed the individual particles of a substance are, the denser the substance is. Understanding density is a useful tool in identifying minerals since substances have different densities. For example, a ruby and an emerald of the same size and cutting style will have vastly different weights. This is because the density of the ruby is greater than that of the emerald. Therefore, a one carat ruby will be smaller in size than a one carat emerald.

In gemology we measure density via the specific gravity (SG) of a gem. Specific gravity is defined as the ratio of the weight of a substance to the weight of an equal volume of water. The SG of a gem can be measured using a scale and water. Specific gravity varies with chemical composition and crystal structure type. Therefore it can be very useful in identifying minerals. More commonly, however, the SG of gems is approximated by comparison using heavy liquids. Gemologists use sets of liquids of known SG values to perform the comparison. The stone (faceted or rough) of unknown identity is immersed in the heavy liquid of know SG. The stone is observed to see if it floats (lighter than), sinks (heavier than) or is suspended in (equal to the weight of the liquid). Depending on the result, the stone may be cleaned and then immersed into a different liquid within the set. This process is repeated until the SG of the stone is obtained or all other potential gem varieties have been excluded by process of elimination. Once the SG is obtained, the gemologist will refer to a gemological properties chart of SG values to match it with the corresponding gemstone.


Another important property used to identify gems is the refractive index (RI). After visual observation, the RI is often the first test a gemologist will perform to identify an unknown gemstone. Refraction occurs when light enters a transparent or translucent material or liquid from air causing its speed and direction to change. We have all observed this phenomenon in life. For example, the underwater portion of a canoe paddle appears at an angle to the handle portion above the waterline. As light enters a gemstone its speed is reduced. This sudden reduction of speed causes the light to bend. Therefore, refractive index refers to how many times faster light travels through air than through an object.

The refractive index of diamond is 2.42. Essentially this means that light travels through air at a rate of 2.42 times faster than it travels through diamond. The slower light moves through a gem the higher the RI will be. The instrument used to measure RI is called a refractometer. Once the RI of the subject stone has been determined the gemologist will refer to a chart of known RI properties to identify the stone.

Because gems of the same variety can have slightly different chemical compositions or structures, there can be slight differences of RI value, so RI is often expressed as a range on gemological data charts. For example, the RI of emerald can range from 1.577 to 1.583. However, each gem type has a distinct RI. Although there can be overlap between related materials, visual observations are sufficient to make the proper identification. For example, the RI of garnet and ruby can overlap. However, the single refractive nature of garnet (light travels at the same speed regardless of direction) is sufficient to separate it from doubly refractive gems (light splits into two beams each traveling at a different rate of speed) such as ruby.

Gemologists use a myriad of other characteristics and properties to identify gem materials. These can include the hardness of the stone, observation of diagnostic inclusions, growth zones and color. The gemologist’s best asset is experience.


Just because a stone is a sapphire or emerald or other gem variety does not automatically mean it has value. Most gem deposits typically produce much more low quality rough than they do rough with gem potential. Minerals such as diamond, corundum and quartz have industrial applications, so even in low quality form they are desirable to mine. But others do not. What are the factors that determine if a piece of gem rough has value?


The measures used to evaluate the quality of a gemstone have been developed over centuries. Ancient writings from China, Egypt, India and the Roman Empire have described the value factors associated with gems over thousands of years. The valuation of gems observed in the industry today has been an evolving process. One common thread in this process has been the importance of color. As is implied in the term colored stone, the single most important factor to a gemstone is color. Some colored stone experts estimate that the color component of a gemstone can account for 50% to 70% of its value.

Gemologists consider a gem’s body color in terms of hue, tone and saturation. Combinations of these properties help create the colors we observe. I say “help create” because color is not an absolute: it is a perception of the interaction between the object, eye and brain of the observer. In other words, color occurs when seen, and the typical human can see about 10 million different colors! Hue is the base color that first stands out when viewing a gemstone. For example, blue, violet, purple, red, orange, green and yellow are all hues. Combinations of these can form additional hues. For example a common color in tanzanite is purplish blue. The Gemological Institute of America identifies 31 base hues that can be used to describe most gems. That chart consists of the 7 base hues listed above plus 24 combinations of the above.

Tone describes the degree of lightness or darkness of a color. For example, a red stone can be dark red, medium red or light red. The color may also be between two tone values, i.e., medium light red. The GIA tone scale has 11 positions ranging from colorless to black. Black and white are not considered hues. They are the extremes of the tonal scale.

Saturation refers to the degree of intensity of a color. The GIA saturation scale groups hues into cool hues, (including blue and violet) and warm hues (including orange and red). Cool hues can look grayish as their saturation of color decreases, while warm hues can look brown as their saturation level decreases. The GIA saturation scale identifies six degrees of saturation. These range from grayish (in the cool hues) or brownish (in the warm hues) to vivid. Desirable saturation levels for most gems fall in the moderately strong, strong or vivid range.

The relationship between hue, tone and saturation is important in valuing a colored stone. The body color that we observe in the gem is a combination of these properties. Usually as a gem becomes more saturated, it will also have a darker tone. While the most valuable gem will be a 6-vivid on the saturation scale, the most valuable gems will not be the highest tone value of 8. These gems are actually too dark in tone. The optimal tone is typically medium to medium dark, 5 to 6 on the tone scale. Different gems have different optimal tone values based on what nature provides us. An example of this is aquamarine, which almost always has a lighter tone of blue vs. sapphire, which often has a dark tone of blue. Both may be the same hue, but the optimal tone expected to see in the market is much different for both. When examining a gemstone, be aware of the light source used to illuminate the gem. Ask yourself if the stone have the same appearance in normal viewing conditions.


Clarity refers to the relative degree to which a transparent gemstone is free of internal or surface characteristics or imperfections. Internal characteristics are commonly referred to as inclusions and external characteristics are commonly referred to as blemishes in the gem trade. The types of inclusions encountered in gems include needles (mineral fibers that formed during crystallization), crystals (small mineral crystals of the same or different type as the gem that formed in the host), pinpoints (tiny crystals), feathers (natural separation across an atomic plane), fingerprints (a healed or partially healed break in the gem), color zoning, chips, scratches, etc.

Inclusions are not necessarily a negative feature to a gem. Certain gems cannot exist without them. Cat’s eye chrysoberyl and star sapphire require needle inclusions to create the desired phenomena. Gemologists consider the type, size, number and position of inclusions when considering clarity. They also consider how noticeable the inclusions are. Gemologists study the type of inclusion because certain inclusions pose a greater likelihood that the gem could be damaged under normal wear than others. They study the size because usually the larger the inclusion the more it may detract from the stone’s appearance or pose a concern for damage. The number of inclusions is considered in relationship to the size of the inclusions. Numerous small inclusions typically will not impact the value of a gemstone to the extent that one large inclusion can. The position of an inclusion refers to its location within the stone. For example, an inclusion positioned under the table facet is likely to be more apparent than one located near the girdle.

Minerals form in different environments and from different chemical elements. As a result, some varieties are more prone to containing inclusions than others. The GIA clarity grading standard for colored stones takes this fact of mineralogy into consideration. Colored gemstones are typed as Type I, Type II or Type III. Type I gems grow extremely clean in nature and usually have no eye visible inclusions. Type II gems typically grow with some minor eye visible inclusions. Type III gems almost always form with many eye visible inclusions. Most popular gemstone varieties fall into either type I or type II. With consideration of the type classification, the clarity is graded as Eye-Clean, Slightly Included, Moderately Included, Heavily Included, or Severely Included. It was stated earlier that color is the most important factor in evaluating a colored stone. It should be noted that if a type I gem (such as aquamarine) has numerous, large eye visible inclusions this will have a significant negative impact on value regardless of how strong and pure the color is.


Here, cut refers not to shape of the stone (round, pear etc.), but to how well the proportions, symmetry and polish are executed. How well a gem is cut can greatly affect the overall value because of its influence on color. Well-cut stones bring out the beauty and brilliance more, thus increasing the overall grade. Poorly cut stones will see light leak out of the stone instead of being returned to the viewer’s eye.

There are many factors to consider when valuing the cut of a gemstone. Brilliance, windowing, extinction, polish and symmetry are all factors that should not be overlooked. Brilliance refers to how bright or glittery a gem is. Windowing refers to the washed out area of body color in a gem. If there is a significant degree of windowing in a gem, one can usually read text placed underneath the stone when the stone is placed in contact with the text. Windowing is common in stones that are cut too shallow. Extinction refers to dark areas in the stone. Extinction is encountered in stones that are cut too deep or have significant pavilion bulge (rounding of the pavilion). Polish and Symmetry refer to how sharply and well defined the relationship is between the crown (portion above the girdle) and the pavilion (portion of the stone below the girdle). When care is taken, all facets will be polished well and the arrangement will align the facets properly for the shape of the gemstone.

When combined, the evaluation of color, clarity and cut result in a total grade that considers these three factors. While no grading standard exists, our own internationally used publication, the “GemGuide” by Gemworld International, Inc., uses terminology of Commercial, Good, Fine and Extra fine to refine this step.


Treatment refers to any process other than cutting and polishing that improves the appearance of the color or clarity, or that are used to alter the appearance (color, clarity or phenomena), durability, value, or supply of a gemstone. Today, most gems are treated to improve appearance. Treatment processes can consist of heat, irradiation, dyeing, oiling, or other processes. Detection of these treatments may be easy to nearly impossible. Treatments should always be disclosed to the consumer. The following is adapted from the American Gem Trade Association Treatment Disclosure Codes.


Clarity enhancement/Fracture filling refers to the filling of surface breaking cavities or fissures with a colorless glass, plastic or similar substance. This process is done to improve durability, color and transparency and may add weight.


Dyeing refers to one of the oldest treatments recorded. The treatment involves the introduction of a coloring agent into a gemstone to give it a new color, intensify an existing color or improve color uniformity.


This process is a form of heat treatment that is typically encountered with ruby. During heat treatment fluxes, or in some instances heat alone, is used to induce the healing of fractures by partial melting and controlled cooling of the stone during which a nutrient rich solution develops in the fractures and crystallizes as the stone cools, thus healing the fracture.


The heat treatment of gems is the most common treatment technique used on gems. Reference to heat treatment of gems is found in gemological literature dating back more than one thousand years. However, widespread use dates back only about fifty years. Unlike most gem treatments, heat treatment is not detectable in most gem varieties including aquamarine, citrine and tanzanite. This treatment is usually detectable in ruby and sapphire. Heat treated gems are stable and the treatment is usually permanent. The vast majority of gemstones are heated to alter their color. In ruby and sapphire the treatment is often performed to improve color and clarity. The alteration usually involves burning out secondary colors present in an attempt to improve the appearance.


Irradiation refers to the use of neutrons, gamma, ultraviolet and/or electron bombardment to alter a gemstone’s color. The irradiation stage of the process is then usually followed by a heating phase to effect the change. Blue topaz is typically produced by irradiation.


Oiling refers to a filling of surface reaching cracks or fissures in a gem with a colorless oil or resin, wax or other substance except glass or plastic, to improve the gemstone’s appearance. The purpose is to diminish the visibly of fractures and thus improve transparency in the stone. The treatment is usually not permanent.

This is a brief list of treatments commonly encountered in the gem trade. It is not all inclusive. Other treatments exist.


Buying a gemstone can be a daunting task. It is an experience that requires trust, not only of our senses in judging what we can discern about the gem one is considering, but also the representations of the person selling the gem. Many factors set value. Some of them like weight are tangible; others like clarity, color or origin are not. Gemstones may be accompanied by a lab report listing the color, cut, clarity and weight. They may also contain a comment about treatments or origin. Certain gemstones such as ruby or sapphire can receive a premium in the market if they are believed to be from a particular country (origin). In lieu of an independent grading report, the seller may offer certain guarantees.

Consumers must reconcile their budget with their tastes. A smaller gem of finer quality will cost as much or more than a larger gem of lesser quality. Nature produces far fewer of the top quality gems, so there is good reason for the greater price. The following is a summary of factors for some of the most popular gems in the trade.


Ruby is generally considered the most valuable of the colored gemstones when priced per carat for exceptional quality gems. It is considered one of the “big three” in the industry ruby, emerald, sapphire. Color: Rubies vary in color from orangy red, red slightly purplish red, strongly purplish red, to red. Clarity: Type II May be eye clean with minor inclusions visible under magnification. Cut: Some native cuts may be found (gems cut at the mine location without high regard to skill in cutting). However, well-cut stones are available. Carat weight: Stones weighing more than 5 carats in finer qualities are very rare. In commercial grades, stones are available from in all sizes up to 10 carats. Reference Data: Refractive Index: 1.762-1.770, Specific Gravity: 4.00 Common Treatments: Most rubies are heat treated to improve color. Some may also be heated in the presence of flux which can leave residue within the ruby, known as flux healing. Another method of filling fissures uses lead glass and leaves this residue inside the gemstone.


Emerald is one of the most valuable gems, prized throughout history. It is considered one of the “big three” in the industry – ruby, emerald, sapphire. Color: Very strongly bluish green, bluish green, very slightly bluish green, green and slightly yellowish green. Clarity: Type III usually has eye visible inclusions. Cut: Some native cuts may be found (gems cut at the mine location without high regard to skill in cutting). However, well-cut stones are available. Carat weight: Stones weighing more than 8 carats are rare in finer quality. In commercial qualities, stones are available in all sizes up to 20 carats or even larger may be encountered. Reference Data: Refractive Index: 1.577-1.583, Specific Gravity: 2.72 Common Treatments: Nearly all emeralds are oiled with a variety of oils, some with resins added. Filling of emeralds may also take place. These treatments help to hide the natural fissures within emeralds which is a common feature of their growth in nature.


Sapphire is the most popular gemstone in the United States. It is considered one of the “big three” in the industry – ruby, emerald, sapphire. Color: Sapphires can be found in a variety of colors in each of the seven primary hues. However, blue is the most common, most desired, and most expensive. Clarity: Type II—May be eye clean with minor inclusions visible under magnification. Cut: Some native cuts may be found (gems cut at the mine location without high regard to skill in cutting). However, well-cut stones are available. Carat weight: Stones weighing more than 20 carats is rare in finer qualities. In lower grades very large sizes are available. Reference Data: Refractive Index: 1.762-1.770, Specific Gravity: 4.00 Treatment: Heat is very commonly used to improve the color of sapphires.


January: Garnet

February: Amethyst

March: Aquamarine

April: Diamond

May: Emerald

June: Alexandrite

July: Ruby

August: Peridot

September: Sapphire

October: Opal

November: Topaz

December: Zircon (now tanzanite on some lists)

Important Gem Stones with pictures

















Cat"s Eye















Golden Beryl










Lapis Lazuli

















Rose Quartz


Rutilated Quartz



Smoky Quartz