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Precious vs. Semi-Precious Gemstones

The rarest gems are classified as precious stones. These include diamonds, sapphires, emeralds, tanzanite and rubies. Precious stones are more expensive than semi-precious but they follow the same color, cut, and clarity rules for value.  All other gemstones are classified as semi-precious. However, today these separations do not have much significance. It is more common to simply classify between diamonds and colored stones.

Colored Stone Grading: Art or Science?

The grading system that is used for colored stones today is modeled after the grading system for diamonds. However, diamonds and colored gemstones do not share all the same qualities and that creates problems when attempting to grade a colored stone. When grading a diamond, the stone's brilliance and fire come into account, while when grading a colored stone these factors are of no importance. For example, in a ruby or sapphire, fractures and inclusions sometimes enhance the quality and value of the stone. Therefore, the grading systems do not work for each other. When looking at a colored stone, you must look at it as a whole and not just a series of parts as one does when grading a diamond. You still use the similar grading system as diamonds however the fifth C is color coverage.

Gemstones and the 5th 'C' - Color Coverage

A gemstone is a mineral, rock or petrified material that after being cut or polished is used in jewelry or as a collectible. When looking for a gemstone, as you would a diamond, you look for the same 4c's, plus one more. You also look for the fifth C, color coverage. However, gemstones do not have a grading system for the 5c's as diamonds do, therefore you must be an informed consumer.

Gemstone Characteristics

Gemstone Color

Color is what attracts us to a gemstone; therefore it is the single most important gem characteristic. Most gems have very attractive colors as their main feature, and are expected to be transparent, but it is not necessary for a stone to be transparent. There are three dimensions to color; Hue (position), Saturation (intensity) and Tone (lightness or darkness).

Gemstone Hue

Gemstones can be found in most colors, which result in various hues or shades for each color. The most valuable gemstones are those that display a more pure form of the color, and only hints of other colors. Gems with hues that most closely resemble the red, green and blue sensors in our eyes are most popular. But there is much about hue that is very subjective and will depend upon an individual's personal taste.

Gemstone Tone

The tone of a gemstone signifies the depth of the color. The degree of lightness or darkness of a color, as a function of the amount of light absorbed. 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 darkness. Note that as saturation increases, so too does tone (since more light is being absorbed).

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. Also view gems at arm's length and look for those that are attractive even at a distance. Exceptional gems will look great under all lighting conditions and viewing distances. 

Gemstone Saturation

The saturation or the intensity of a gemstone refers to the color purity and the degree to which the gem is free from brown or gray hues. Gemstones that show very little gray or brown are the most popular and are described to have "vivid" or "strong" color saturation. We tend to be more attracted to highly saturated colors.

Gemstone Clarity

Gemstone clarity is a major factor in determining quality and, consequently, the value of a gem.

Gemstones tend to form under quite unique circumstances and each stone is comprised of different minerals. Therefore, the minerals leave a distinctive set of marks and inclusions. In the case of a gemstone, inclusions will not necessarily detract from the beauty of the stone. When you are examining the gemstone's clarity you must look at the gemstone as an individual stone. Some gems have naturally fewer marks or inclusions, while some have more than others. The best values are available in gemstones that are moderately included. Exceptional gemstones with few or no inclusions can be found, but they can command much higher prices.

Gemstone Cut

Colored gemstones are usually cut in order to showcase the beauty of the stone's color. When a gem is well-cut it maximizes the color of the stone, minimizes its inclusions, and displays excellent symmetry and proportion.

Cutting standards are far more relaxed for colored gemstones than for diamonds. There are no real geometrical standards for maximizing the brilliance or color of the gem because there are so many different colors of gemstones. Rare gemstones are at times cut for size without regard to their color because it is so hard to find something with that hue.

It is important to examine the symmetry of the gemstone in its setting. A gemstone that is well-cut is symmetrical and reflects light evenly across the surface of the stone, the polish is smooth and there are no nicks or scratches. When a stone is cut asymmetrically it indicates low-quality.

Gemstone Carat Size

When looking at the carat weight of a gemstone, you should keep in mind that the weight does not necessarily allow you to properly envision the actual size of the gemstone.

Because different gemstones have different densities or specific gravity, two gemstones that appear to be the same size may actually have very different weights. In the jewelry industry it is common to refer to millimeter size and then convert to carat weight. It is very important to ask about the dimensions of a gemstone to make sure that the majority of the weight of the stone will be seen when set.

Gemstone Color Coverage

When dealing with gemstones, it is not enough to only discuss the color, hue, position, saturation and darkness. In addition, we must also discuss the color coverage, scintillation and dispersion. When there are differences in proportions, inclusions, transparency and fluorescence, it can produce a vast difference in the color coverage.

There are a few factors that tend to influence color coverage. They include proportions, fluorescence and inclusions. Suitable cutting is also crucial in maximizing color coverage. When a gem is cut too shallow, all short light paths are apparent, which reduces saturation in many areas. When that occurs, the areas are called "windows". In addition, when the gem is cut too deep, it allows light to exit the sides, which creates dark areas called extinction. When there is an area that permits total internal reflection, it displays the most highly saturated colors which are called "brilliance".

When shopping for a gemstone, the qualities are similar to a diamond. However, you should still be aware of the differences in order to suit your taste and budget.

Gemstone Durability and Hardness

If you are looking for a suitable gemstone, durability is a must. Gemstones vary in terms of their durability, some are indestructible and some are much more delicate and need to be taken care of. However, something that seems to be very fragile can also be very strong.

A common misconception is that durability and hardness is the same thing, but they are not. The main quality for gems is that they must be resistant to abrasions. The scale that determines hardness is the Moh's hardness scale; it lists ten minerals and lists them on a scale from 1 to 10. Each numbered mineral has the potential to scratch the mineral listed below it. A diamond is a 10 on the Moh's scale and is the hardest known natural substance. The mineral that comes in at a 9 on the Moh's scale is that of corundum, which is what rubies and sapphires are made from. A topaz is an 8, which makes a topaz easier to break. Quartz, which is what amethyst and citrine are made of, is a 7 on the scale and is the softest translucent material. The scale goes down in number to include the remaining minerals and gemstones.

Here is a list of minerals from hardest to softest along with their absolute hardness number:

Moh's Scale


Absolute Hardness



absolute hardness 1500



absolute hardness 400



absolute hardness 200



absolute hardness 100


Orthoclase Feldspar

absolute hardness 72



absolute hardness 48



absolute hardness 21



absolute hardness 9



absolute hardness 2



absolute hardness 1

Gemstone Transparency

Technically known as "diaphaneity", the degree of transparency of a gemstone is one of its most directly observable and familiar characteristics.

In terms of colored gemstones, transparency does not have to be an important factor. Sometimes, when a colored stone has inclusions the color is enhanced. Most transparent gemstones are cut into faceted stones.

Gemstone Luster

The luster of gems is one of their important and distinctive characteristics. The luster of a gem is produced by the light which it reflects back to the eye, and this may vary in quantity and quality with the nature of the surface. Depending on the different gemstone, the way light affects the color is different for every stone. Some stones do not change color in various light and some do drastically. It is important to examine the stone in different lighting in order to gain the best perspective of the stone's potential.

Gemstone Pleochroism

Color changes which are evident when viewed from different angles in gemstones (iolite, alexandrite, and andalusite) are called pleochroism. It is very important for the gem cutter to cut a pleochroic stone properly in order to show off the different colors. Pleochroism literally means ?many colored'.

When a gemstone is pleochromatic, it means that when light is reflected, different colors appear in the stone. Ruby and sapphire have two color shades and are pleochroic; in ruby, for example, yellow-red and purplish-red, which distinguishes it from garnet and red spinel, which has no pleochroicism. Iolite displays lavender-blue, gray, and pale yellow when viewed from different angles.

Gemstone Enhancements

Gemstone enhancements are any treatment or process other than cutting and polishing that improves the appearance, durability, value or availability of a gemstone. A gemstone treatment or enhancement is considered permanent if the effect of the enhancement does not change under normal wear or display. Rubies, emeralds, and sapphires usually undergo treatments to enhance color and fill inclusions. Most sapphires and some rubies are heat-treated to improve color. This process dissolves trace elements that are already in the stone. This treatment is a permanent enhancement and is accepted in the jewelry trade. The jewelry industry considers treatment of these three gemstones standard practice.

Most gemstones available today have been enhanced or treated in some way. Heat treatment is a common enhancement that has been used for centuries. The jewelry industry recognizes heating and hardened resins as acceptable permanent treatments for most gemstones.