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Gemspedia >> Gems >> Understanding Gemstone

Understanding Gemstone

The beauty, rarity and historical mystique of gems are timeless. Their richly diverse varieties and colors come in a kaleidoscopic array. However, before you make a purchase, you will need a basic understanding of gems.

Understanding and appreciating the value of gems is essential to making a successful and rewarding purchase. Regardless of the gem variety you’re buying, there are a few constant rules to bear in mind.


What is a gem?

Gems are generally defined as naturally occurring materials (generally minerals) that are used in jewelry or for personal adornment. Generally, most gems possess the attributes of beauty, rarity and durability.

You may have heard about the 4 C’s related to valuing gemstones: color, clarity, cut and carat weight. While gemstone professionals and connoisseurs the world over rely on these factors, we’ll also cover other elements that also need to be taken into consideration when purchasing colored gemstones.


Color: The First C

Understandably, color is the single most important factor when evaluating colored gems. Generally, the more attractive a gem’s color, the higher the value. Bright, rich and intense colors are generally coveted more than those that are too dark or too light. However, there are exceptions, such as the lovely padparadscha sapphire, which is valued for its delicate pastel hues.


The three dimensions of color. Hues change as one goes around the wheel; tone changes in the vertical axis, with the lightest tones at the top, while saturation increases from the center out on the horizontal axis.

Color is typically described in three dimensions: hue, tone and saturation.


Hue position. The position of a color on a color wheel, i.e., red, orange, yellow, green, blue and violet. Purple is intermediate between red and violet. White and black are totally lacking in hue, and thus achromatic (‘without color’). Brown is not a hue in itself, but covers a range of hues of low saturation (and often high tone). Classic browns fall in the yellow to orange hues.


Generally speaking, gems with hues that most closely resemble the red, green and blue (RGB) sensors in our eyes are most popular. Thus the colored gem trinity, ruby,emerald and sapphire. But there is much about hue that is a personal preference and will depend upon an individual’s personal taste.


Saturation (intensity). The richness of a color, or the degree to which a color varies from achromaticity (white and black are the two achromatic colors, each totally lacking in hue). When dealing with gems of the same basic hue position (i.e., rubies, which are all basically red in hue), differences in color quality are mainly related to differences in saturation, because people tend to be more attracted to highly saturate colors. The strong red fluorescence of most rubies is an added boost to saturation, supercharging it past other gems that lack the effect.


Three green gems, showing a variation in hue position. The center stone is a straight green, while the stone at left is a more bluish green and the stone at right a slightly yellowish green. Generally speaking, hue position is of less importance than saturation in judging quality

Tone. The degree of lightness or darkness of a color, as a function of the amount of light absorbed. White would have 0% tone and black would be 100%. At their maximum saturation, some colors are naturally darker than others. For example, a rich violet is darker than even the most highly saturated yellow, while the highest saturations of red and green tend to be of similar tone. Note that as saturation increases, so too does tone (since more light is being absorbed). However, there reaches a point where increases in tone may result in a decrease in saturation, as a color blackens.


When judging the quality of a colored gem, tone is an important consideration. Before buying, it’s always a good idea to consider the lighting conditions under which it will be worn. Look for stones that look good even under the low lighting conditions you find in the evening or in a restaurant, for these are typically the conditions under which fine gems are worn and viewed. Also view gems at arm’s length and look for those that are attractive even at a distance. Exceptional gems tend to look great under all lighting conditions and viewing distances.

The relationship between tone (vertical axis) and saturation (horizontal axis).

Although specific colors are more popular than others, personal preferences are also important. The colors seen should ideally remain attractive regardless of prevailing light conditions. Whether viewed indoors, outdoors, by day or by night, a gem should always remain beautiful.


The mixing of color hues into combinations, such as violet-blue in tanzanite and bright blue green in apatite is attractive and value-enhancing. Many gems also have specific expressions to denote the very top colors found within a species (e.g., cornflower blue sapphire). Such terms relate to the most desirable colors to be found within a gem species.


In regards to coloring agents, minerals are termed idiochromatic (self colored) if the color results from an element that is a basic part of the mineral’s chemical formula. If the color is due to an impurity or some other cause, the mineral is termed allochromatic (other colored).


Clarity: The Second C

Often adding character and individuality, most gems contain tiny natural features called inclusions. Mostly microscopic in nature, they are best viewed under magnification. Inclusions are a fascinating hallmark of authenticity that record a gem’s natural relationship with the earth.


Clarity is judged by reference to the visibility of inclusions and their impact on durability. Note also that cabochon-cut gems generally have lower clarity than faceted gems. This is because inclusions are more visible in faceted stones than in cabs.


Generally, the higher the clarity, the higher the value of the gem. Magnification is often used to locate inclusions but, with the exception of those that might impact durability, inclusions that don’t interfere with the brilliance and sparkle of a gem don’t affect the value. In this way, colored gems are quite different from diamond. Indeed, in certain cases, such as with Kashmir sapphires and demantoid garnets, the inclusions actually enhance beauty and value.

Inclusions such as these rutile needles in ruby are often strikingly beautiful. Gemoloigsts study inclusions for clues about the origin and nature of gems.

In transparent gems, the degree of transparency and light return (brilliance) is considered crucial. However, through market experience, we learn to expect certain degrees of clarity from certain gems. For example, aquamarine is generally expected to be clean with no eye-visible inclusions and yet emerald is held to a lower standard because it occurs in nature with lower clarity levels.

Cut : The Third C

The function of the cut is to display the gem’s inherent beauty to the greatest extent possible. Since this involves aesthetic preferences upon which there is little agreement, such as shape and faceting styles, this is the most subjective of all aspects of quality analysis.


Unlike diamonds, colored gems possess variable optical properties and thus are not often cut to a uniform ideal. A well-cut colored gem exhibits even color, an acceptable number of inclusions for the type and good brilliance. As gemstones are nature’s creations, cutting is often a juggling act between carat weight retention and beauty.

In well-cut gems, most light returns as brilliance (left). If a gem is cut too shallow, light will pass straight through, rather than returning to the eye as brilliance. This is termed a “window” (right).

Broadly, the styles of gem cutting can be divided into faceted gems (those with geometrically-shaped flat polished faces) and non-faceted gems (those that do not have geometrically-shaped flat polished faces, such as cabochons).


In essence, the cut should display the gem’s beauty to best advantage, while not presenting setting or durability problems. If a gem is beautifully cut, things such as depth percentage or length-to-width ratio matter not a bit. What works, works. The eye, the mind and the heart are the final arbitrators, not numbers.


One final note about cut. The most expensive colored gems (particularly colored diamonds and rubies) often feature misshapen proportions and symmetry. This is because the value of the material is so high that the cutter strives to save every point in weight.


Carat Weight: The Fourth C

Gemstone weight is measured in carats. This archaic unit of measurement originates from the traditional use of carob seeds to weigh gems in the bazaars of the Middle East and Asia. Carob seeds were used because of their consistent size and shape. In 1907 in Europe and in 1913 in the USA, carat weight was standardized as one fifth of a gram (one carat is the equivalent of 0.20 grams). Gemstones less than one carat are often measured by dividing the carat weighed into 100 smaller units known as points. For example, a 50-point gemstone can also be described as half a carat and a 25-point stone can also be described as a quarter of a carat.


The term carats is often confused with karats. Karat is a measurement of gold purity and has no relationship to the term carats, other than the fact that they both originate from the use of carob seeds, which in this case was used to weigh the alloys added to gold.



One three-carat gemstone is usually worth more than three one-carat gemstones

Generally, as the weight of a gem increases, so does its price per carat. Large gems are rarer than smaller ones, so per carat prices rise exceptionally. For example, a three carat ruby is always worth far more than three one-carat rubies of the same quality. The only time the combined weight of smaller gems costs more than a singular gem of the same carat weight is when the labor cost of applying all the facets to the individual gems outweighs the difference in price.


Gemstone prices also increase rapidly when in excess of certain key weights. For example, a 2.01-carat ruby has a higher price tag than a 1.99-carat ruby, despite a negligible difference in actual size. Gem pricing is therefore based on a nonlinear scale of increments. To put this into context, a 16-carat ruby sold at Sotheby’s in New York in October 1988 for a staggering $3,630,000. If gem pricing was linear, that would make a similar one carat ruby worth $226,875.


As gemstones are nature’s creations, no two gemstones are absolutely identical. If you are buying a gem with a rounded carat weight, make sure that the quality of the cut (i.e., beauty) has not been compromised to achieve a larger carat weight, as a reduction in beauty might reduce the value of the gem.


Color Coverage: The Fifth C


Moderate color zoning in tourmaline.

With gems, we are not dealing with opaque, matt-finish objects of uniform color. Thus it is not enough to simply describe hue position, saturation and darkness. We must also describe the color coverage, scintillation and dispersion.


Differences in proportions, inclusions, transparency, fluorescence, cutting, zoning and pleochroism can produce vast differences in the color coverage of a gem, particularly faceted stones. A gem with a high degree of color coverage is one in which color of high saturation is seen across a large portion of its face in normal viewing positions. Tiny light-scattering inclusions, such as rutile silk, can actually improve coverage, and thus appearance, by scattering light into areas it would not otherwise strike. The end effect is to give the gem a warm, velvety appearance (Kashmir sapphires are famous for this). Red fluorescence in ruby boosts this still further.


Proper cutting is vital to maximize color coverage. Gems cut too shallow permit only short light paths, thus reducing saturation. Such areas are termed windows. Those cut too deep allow light to exit the sides, creating dark or black areas termed extinction. Areas which allow total internal reflection will display the most highly saturated colors. These areas are termed brilliance.


The effects of pleochroism in green tourmaline. Along the vertical axis, a bluish green color is seen, while along the horizontal axis a more yellowish green color is found. This is a product of the doubly refractive nature of tourmaline.

Color zoning can also reduce color coverage. Ideally, no zoning should be present. Moderate to severe color zoning does impact quality, and thus price. Color zoning is always judged in the face-up position, in an 180° arc from girdle to girdle, with the gem rotated through 360°. Zoning visible only through the pavilion generally does not impact value.


Pleochroism is noticeable face-up in some gems, such as some tourmalines and iolite. It typically appears as two areas of lower intensity and/or slightly different hue on opposite sides of the stone.


In summary, a top-quality gem would display the hue of maximum saturation across a large percentage of its surface in all viewing positions. The closer a gem approaches this ideal, the better its color coverage.


Country of origin

The sixth C is country of origin. Names of geographical locations should only be used when they denote the areas from which gemstones originate (for example, it is misleading to call a high quality emerald ‘Colombian’ if it doesn’t actually come from Colombia).


While there are exceptions, gemstones that are rich in history and folklore are generally more prized than those lacking historical connotations.



“The love of precious gemstones is deeply implanted in the human heart,” wrote George Kunz in his book, The Curious Lore of Precious Stones:
“The cause of this must be sought not only in their coloring and brilliancy but also their durability.” Kunz further wrote, “The sheen and coloration of precious stones are the same today as they were thousands of years ago and will be for thousands of years to come. In a world of change, this permanence has a charm of its own that was early appreciated.”


A gemstone must be durable enough not to break or fade over years of wear. Its brilliance and beauty are expected to last for a very long time, even to the point where a gemstone will outlast its owners and be passed on to sons and daughters, who, in turn, will help maintain its status as a gem, by awakening appreciation in succeeding generations. While gems with better durability and resistance to wear are generally more highly prized than those of lesser durability, given proper care, all gemstone jewelry should be suitable to be passed down to many generations.


Durability is a combination of three properties:


This is the ability of a gem to resist surface scratching. The hardest natural substance is diamond. Second to diamond is corundum (ruby and sapphire) and third is topaz. Hardness is quantified on a scale of one to ten, with ten being the hardest and one the softest. It is a comparative not a relative scale, the minerals chosen set the levels of hardness. The system was devised in the 18th century by a Viennese mineralogist Friederich Mohs and is named after him (Mohs’ hardness scale).


Diamond may be the hardest gem, but it is not the toughest


Jade is an example of a gemstone that is not particularly hard, but extremely tough

This is the ability of a gem to resist the development of fractures (random non-directional breakage) or cleavage (splitting along certain well-defined planes).


This is the ability of a gem to resist physical or chemical damage.



By their very definition, all gems are rare. Rarity can be described in three, often unrelated, ways:

  • Geological

  • Marketplace

  • Comparative

While scarcer gems are generally more highly prized than less scarce varieties, geological rarity doesn’t always mean a gem has a higher value in the marketplace and vice versa. Beauty and marketing play a big part. Sometimes the geological rarity of a gem type jeopardizes commercial viability. Tsavorite garnet is rarer than emerald, and is sometimes more beautiful, but because of its rarity, it cannot compete with emerald in terms of the consumer perception of its value. Given the enormous diamond stockpiles and new sources springing up around the world, when compared to many colored gemstones, diamonds are not especially rare. Strict control of polished diamond supply in the market, combined with sophisticated consumer advertising, has elevated diamonds to the extent that they are perceived as a rare and coveted product.


If a gem variety is so rare that it is essentially unknown to the general public, it is often classified as an exotic or collector’s gem. Gems such as boracite, childrenite and simpsonite are extremely rare, attractive and durable, but they are unlikely to command prices appropriate to their rarity because few people are aware of their existence.


Pairs and suites

Pairs or suites of gems matched for color, clarity and cut are valued more highly per carat or per gem than single gems of the same quality. Given the rarity of many gems, a matching set is disproportionately hard to find and will command a higher per carat price than if each of the gems from the suite were sold separately.

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