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Quality Guide To Colored Gemstones

First, what is a gemstone? We know that there are literally thousands of minerals on the earth and we know that gemstones are minerals, so how do gemologists (people who study gem materials) decide which ones qualify as gemstones? I will share an overview of this fascinating topic, as well as what you should keep in mind as you shop for colored gemstone jewelry.

Criteria for Identifying Gemstones:

To qualify as a gemstone a mineral must possess three qualities:

1) Beauty – The most desirable gemstones possess pure colors with minimal secondary colors that do not detract from their beauty. For instance, brown or gray casts within gemstones reduce the purity of the color; so a brownish-red or grayish-purple is generally not considered attractive. Gemstones must also be free from large internal and external characteristics (flaws or damage) that are easily visible. And, finally, gemstones must be cut to maximize light return (liveliness/fire) and show the best color when viewed from the top.

2) Rarity – Gem minerals should be rare enough to be desirable, but not so rare that they defy recognition or commercial viability. Gemstones are highly valued because of their rarity. Some colored gems are either extremely rare or extremely difficult to locate and mine; this translates into limited availability and premium prices. E.g., Top quality rubies larger than five carats are extremely rare and will be far more expensive than a comparable size diamond.

3) Durability – Finally, gemstones should be durable enough to set into jewelry without fear of damage or if worn frequently.

Durability has three components;

  • Hardness: A gemstone’s ability to resist scratching
  • Toughness: A gemstone’s ability to resist chipping or breaking
  • Stability: A gemstone’s resistance to fading or color change when exposed to heat, prolonged strong light, or chemicals.
Mohs’ Scale of Hardness

So, within the mineral world, there are many that are attractive, e.g. sulfur, an attractive, bright yellow, but it lacks both hardness and toughness, so fails the gem-qualifying criteria. Not to mention the off-putting odor.

It is critical to be familiar with the durability characteristics of the specific gemstones you are considering buying as they all differ when it comes to durability. For instance, when I sold engagement rings, many women asked for Tanzanite, because of its gorgeous bluish-purple color. As disappointing as it was, I had to offer them alternatives due to the mediocre hardness rating of 6.5 and poor toughness. It is also brittle because it is usually heated to improve color. Unfortunately, with these combined conditions Tanzanite would not be suitable in engagement rings as they are normally worn every day. Rings featuring gemstones are the most vulnerable so understanding the durability of the gemstone(s) will help you understand when to wear and how to care for your piece.

The 4Cs of Colored Gemstones

COLOR –When we look at the 4Cs of diamond grading, “cut” is the most important of the 4Cs. With colored gemstones, color is of course the most important, in it is paramount. There are certain elements to be considered when choosing colored stones.

The stones should be vibrant, not grayish or brownish. For example, a bright pure red would be high saturation; brownish-red is low saturation. Pure, saturated colors are rare and therefore sell at premium prices. The stones should not be too light or too dark, medium to medium-dark, saturation, evenly distributed pure color. It is this saturated, evenly distributed, pure, and brilliant color that is deemed high quality in colored gemstones.

  • Hue: The basic color of the stone.
  • Tone: The lightness or darkness of the color.
  • Saturation: The intensity of color, its strength or purity. Look for pure, saturated color with as little modifying (secondary) color as possible.

CUT – Factors that contribute to the beauty

  • Well-proportioned and symmetrical with good shape appeal
  • No washed-out areas where no color is present at all
  • No black areas that replace the primary color

There are no widely used grading standards to judge the cut of colored stones, as there are for diamonds, Colored gemstones should be symmetrical and pleasing to the eye. In the face-up position (as it is seen when worn in a piece of jewelry.) The stone must be brilliant and lively the cutter must determine which direction to cut the crystal in order to obtain the best color, while retaining carat weight.

Poor cutting may result in a stone that appears washed-out and lacks color, seen when one looks through the top of the stone or areas within the stone that are black. Both these characteristics produce stones with an uneven color distribution that are less attractive, lack brilliance, and just look dull.

“Cut” may also refer to how a gemstone is polished. The two styles you will see most often, are faceted – flat polished surfaces that cover stone, or cabochon – a smooth domed top without facets.


Internal Characteristics (Inclusions/flaws)

Internal characteristics are defined as: breaks in the stone, or foreign materials, often other mineral crystals present in the growing environment that are trapped within the stone as it crystallizes.

There is no specific clarity grading system used throughout the industry for colored stones as there is for diamonds. The Gemological Association of America divides gemstones into three categories, based on the amount of inclusions present. This classification system is not familiar to consumers. We expect to see internal characteristics in most colored stones, and if eye-visible and distracting, lower the value of the gemstone.

The same system of weight measurement that is used for diamonds is used for faceted colored stones, that is the carat.

A carat is subdivided into 100 parts known as “points.” 100 points = 1.00 Carat

Not all gemstone jewelry will have the carat weight indicated on the price tags. Beads are measured in millimeters and not by carat weight, opaque gem materials and certain lower-priced gemstones that are abundant are not priced by the carat weight, but rather by the stone.

What Are Gemstone Treatments or Enhancements?

Treatments and enhancements are performed to improve the color, clarity, or durability of gem materials. The enhancements below are those that are accepted by the jewelry industry at large.

Bleaching: The use of chemicals or other agents to lighten or create a uniform color:

  • Cultured Pearls Hardness: 2.5 – Toughness: Poor

Dyeing: The introduction of coloring agents into a gemstone to improve or alter color:

  • Black onyx – to create color Hardness: 6.5-7.0 – Toughness: Good
  • Cultured pearls – organic pink dye to improve rosé overtones Infusion Hardness: 2.5 – Toughness: Poor

Filling: Injecting a gem material with oil to improve appearance: Similar to the improved appearance of a wood table once furniture polish has been applied.

  • Emeralds – filling tiny fissures on the surface with colorless oil, or sometimes tinted oil. The oil will eventually dry out, but treatment can easily be repeated. Hardness: 7.5 – Toughness: Poor to Good

Heating: The use of heat to alter the color and/or clarity of a gemstone – here are a few examples of gemstones that are heat-treated.

  • Sapphires (all colors) and rubies, to improve clarity and color · Hardness: 9.0 – Toughness: Excellent
  • Agate: to improve color Hardness: 7.0 – Toughness: Good
  • Amethyst Quartz and Citrine Quartz: to improve color Hardness: 7.5 – Toughness: Good
  • Aquamarine: to improve color Hardness: 7.5 – Toughness: Good
  • Tourmaline: (Blue, Green) to improve color and clarity Hardness: 7.0 – 7.5 – Toughness: Fair
  • Tanzanite: to improve color Hardness: 6.5 – Toughness: Fair to Poor
  • Rubellite and Pink Tourmaline: to improve color Hardness: 7.0 – 7.5 – Toughness: Fair

Irradiation: Used to improve color.

  • Pink Tourmaline: to improve color Hardness: 7.0-7.5 Toughness: Fair
  • Blue Topaz: to impart color Hardness: 8.0 Toughness: Poor

Surface Coatings: Such as paraffin wax, adds a layer of protection for porous gem materials.

  • Opals Hardness: 5.5-6.5 Toughness: Fair to Poor
  • Turquoise Hardness: 5.0-6.0 Toughness: Poor
  • Lapis Lazuli Hardness: 5.0-6.00 Toughness: Fair

As mentioned, the treatments above are industry standard and are not meant to deceive consumers. There are treatments that are used to trick potential buyers into thinking that the gem(s) they are looking at are indeed completely as Mother Nature created them, or are in better condition than they really are. For example, deep cracks that have been hidden by fillers and not disclosed to the customer are deceptive. A discussion of all the treatments that are performed to deceive is beyond the scope of this article. But to avoid deceptive treatments, buyers should purchase from reputable jewelers.

Rare valuable gemstones, such as large rubies, sapphires, and emeralds, may be accompanied by reports from gemological laboratories stating whether the gem has been treated and if so, the type of treatment. Rare gemstones, such as large rubies, blue sapphires or emeralds that have not been treated are exponentially more valuable and expensive, but must be substantiated by a gem laboratory report.

Caring For Your Colored Gemstones

The safest method of cleaning any gemstone is to use a soft brush (soft bristle toothbrushes are best) along with warm water and a mild liquid detergent, such as dishwashing liquid. Be sure to clean inside the open setting to remove dirt and grease that may be on the stone(s.) You cannot damage any gem material using this method. Avoid chemical jewelry cleaners. To ensure safety, ultra-sonic machines should be used for diamonds only.


Colored gemstones are a marvel to behold, all of them. They should not be labeled precious or semi-precious. In fact, the term semi-precious was introduced in the U.S. primarily to avoid paying import taxes on gemstones other than diamonds, rubies, sapphires, and emeralds. “Semiprecious” just means that the gemstones are more abundant than “precious” gemstones, it does not mean that they are less beautiful.

When it comes to colored gemstones my pet peeve is the concept of “birthstones.” People feel obligated to purchase their assigned gemstone, for themselves or gift a person, even if they don’t particularly like its color. Some people even feel guilty. What’s most important is that you buy the colored gemstones you love and enjoy them.

Just in case you are curious about your birthstone, here they are. But feel free to fall in love with any or all of them!

Birthstone Chart

Thank you for reading my article. I would love to hear your questions, so please leave them below.


Francesca de Granville, G.G., F.GA.

This Post Has 2 Comments

  1. Aly

    I am a huge fan of bright pops of color and there is no better way to see this than in a sparkling, colored gemstone! I am fortunate to really like my birthstone, but I have not let that stop me from collecting pieces that are different from my “assigned” stone. I appreciate all the interesting and helpful information, but I have a question as well. What are your favorite places to purchase gemstones? Do you purchase the stones by themselves?

    1. admin

      Hi Aly, Thank you for your comments!  Yes, I do buy loose gems from time to time.  The best place is at gem shows.  The biggest is in Tucson every year at the beginning of February. The show takes over every meeting facility and hotel room. Thousands of dealers come from all over the world. Your eyes will pop out of your head, Lol. The prices are great because there is so much competition. Everything you could possibly dream of is there. From tables covered with paper plates brimming with loose stones, to carved gemstone pieces from the best carvers in the world, Brazil and Germany, to finished jewelry. Mind you there are lots of guys walking around with machine guns…Also, the biggest jewelry show is in Las Vegas in June, also super fun. Plus you can learn so much from these experts. A great vacation spot in February. Thanks again for reading my post!

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