Hardness is the gem's resistance to being scratched. This is one of the ways that a buyer of rough gems will use to determine what type of gemstone they are looking at. The Mohs hardness scale was developed in the early 1800s by German mineralogist Frederich Mohs. From softest to hardest, here is his list:
- 1 - Talc
- 2 - Gypsum
- 3 - Calcite
- 4 - Fluorite
- 5 - Apatite
- 6 - Feldspar
- 7 - Quartz
- 8 - Beryl
- 9 - Corundum (Ruby, Sapphire)
- 10 - Diamond
Here are some other items you'll recognize and their Mohs scale hardness for comparison:
- 2.5-3 - gold, silver
- 4-4.5 - platinum
- 4-5 - iron
- 5.5 - glass
- 7+ - hardened steel
Gemstones come in two structures, crystalline and amorphous. The shape or morphology of the gemstone is important in identifying the gem and determining how it can be cut for presentation.
- Crystalline structure: A crystal is a solid object with an orderly three dimensional arrangement of molecules repeated throughout the entire volume of the piece. Crystalline gems are single crystals. Rubies, Emeralds and Diamonds are crystalline gems. Since large single crystals are rare, size really does matter in the value of the stone.
- Amorphous structure: Amorphous gems will contain a mixture of components and can include crystals, yet the stone itself is not a crystal. Jade and Turquoise have amorphous structure. The craftsman carefully considers the components of each amorphous stone when deciding on how to best fashion it to bring out it's greatest beauty.
Refraction, dispersion, diffusion, reflection, iridescence, opalescence; no matter what you call it, the way light travels through a gem determines it's characteristics. Each gemstone has unique qualities that cause the light to travel through it in it's own unique way. These individual effects are properties gemologists use to identify and classify each stone. They are also important to the buyer. You might not use the term refraction, but you do enjoy rich color and the way the gemstone sparkles on your finger.
Pricing gemstones can be a very interesting art. You can start by trusting your instincts. That means if a particular gemstone ‘calls your name’, you might want to listen. At the same time there are many more factors to consider. The value of a gemstone starts with it’s grade. We discuss this, in the grading section.
The grade includes:
Color – Generally the most intense and vivid colors in a type of gemstone, will command the highest price.
Cut – Faceted gems should sparkle throughout and have a pleasing shape.
Clarity – Provided that they are not too distracting, clarity features are accepted in most gemstones. In fact they provide assurances that the gemstone is indeed natural.
Carat Weight – Many gemstones come in a variety of sizes, however for more rare materials, the larger the gemstone the higher the price. In many cases the price increases exponentially as they increase in size.
In addition to the factors above, we must include treatments, because how a gemstone is enhanced can sometimes greatly influence it’s value. For example, a non-heat treated Blue Sapphire, over 5 carats, may command a 30%-40% premium over a heat treated Sapphire of the same weight.
Finally, the value of a gemstone is influenced by natural rarity and the law of supply and demand. That is why two beautiful gemstones that look similar can differ substantially in price.
Valuing a gemstone is not always easy, that’s why it makes sense to follow the guidance of a trusted professional when you make your fine gemstone purchase.
Grading colored gemstones can be somewhat subjective. Compared to Diamonds, grading colored gemstones involves many more factors. There are dozens of types of gems along with their corresponding colors. At the same time, there is a basic grading system that we have chosen to follow. It is the general guideline taught by the Gemological Institute of America (GIA).
Hue is the basic color such as red, orange, yellow, blue, green or violet.
Tone describes the lightness or darkness of the hue. This is measured on a scale of 0-10. A zero would be colorless and a 10 would be black. Most stones fall between a 2 which is very light and an 8 which is very dark.
Saturation is the intensity of color. The lower the saturation of color in a stone, the more grayish or brownish it appears. The greater the saturation, the more vivid the color.
Grading Color - Color is the most influential determination when it comes time to price a gem. It can determine up to 70% of the value of the stone.
Few gemstones are used in their raw or rough form. Most are cut and polished before being set into jewelry. The two main stone cut classifications are cabochon and faceted.
Gems like Opal, Turquoise, and Star Sapphires are commonly cut as cabochons. This cutting style is designed to show the stone's rich color, surface properties or phenomenon. Grinding wheels, water and polishing agents are used to grind and polish the smooth, dome shape of the stone.
Transparent gems are normally faceted. This method reveals the optical properties of the stone's interior to its best advantage by maximizing reflected light or what we experience as sparkle. The facets must be cut at the proper angle, which varies depending on the optical properties of the gem type. If the angles are too steep or too shallow, the light will pass right through and not be reflected. Faceting a gem requires precision and artistry, a combination that fully displays the crystal's inner beauty.
Color is the most obvious and attractive feature of colored gemstones. In fact up to 70% of the value of a gemstone is wrapped up in it's color. The color of any material is due to the nature of light itself. Daylight or white light is actually a mixture of different colors of light. When light enters a material, some of the light may be absorbed, while the rest passes through. The part that is not absorbed reaches the eye as white light minus the absorbed colors. A Ruby appears red because the material absorbs all the other light colors except red.
The same material can exhibit different colors. Let's look at Corundum. Ruby and Sapphire are both Corundum and therefore have the same chemical composition but exhibit different colors. Even the same gemstone can occur in many different colors. The best example is the Sapphire. These beautiful gems can be found in just about every color of the rainbow.
This difference in color is based on the atomic structure of the stone. Although the different Sapphires may have the same chemical composition, they are not exactly the same. There can be minute differences at the atomic level. These differences are called trace elements and these miniscule trace elements can be sufficient to change the appearance of color. Some trace elements might absorb certain colors while others have no noticeable effect. Beryl is another example. Beryl is colorless in its pure mineral form. It becomes Emerald green with the trace element Chromium. Replace Chromium with Manganese and Beryl becomes pink Morganite. With iron Beryl is now Aquamarine.
Gemstones are often treated to enhance the color or clarity of the stone. Depending on the type and extent of treatment, this can affect the value of the stone.
- Not Enhanced - These gemstones are ones that have not been treated in any way other than being cut or carved. Un-enhanced gemstones are becoming increasingly rare. This is the result of greater scientific breakthroughs, allowing more and more gems to respond favorably to the treatment process. It is also due to the fact that the finest gemstone material that doesn't need treatment, is now very scarce.
- Heat - Heat can improve gemstone color and or clarity. The heating process is well-known to gem miners and cutters alike. With many types of gemstones heating is a normal part of preparing the gemstones for sale. Most Citrine is made by heating Amethyst. Aquamarine is routinely heated to remove yellow in the stone. This results in a more pure blue. Nearly all Tanzanite is heated to remove green and brown undertones, resulting in a more desirable blue/purple color. A considerable portion of all Sapphire and Ruby is heated to improve both color and clarity.
Often a number of factors are involved in combination to ensure success in heat treatment. These include, temperature control, duration of the heating and cooling as well as atmosphere and the pressure used.
Sometimes the line between heat treatment and fracture filling becomes blurred. Some Burmese Rubies are heated packed in borax powder. When the gem is heated, the borax melts and reacts with the Ruby material causing surface reaching fractures to be filled. This partially heals the fractures and makes the Ruby appear more transparent.
- Irradiation - Irradiation involves bombarding the gemstone with X-rays or gamma rays or with subatomic particles. Virtually all Blue Topaz has been irradiated to change the color from white to blue. Most Greened Quartz is also irradiated to achieve the yellow-green color, widely prized as Oro Verde. Also light colored Tourmaline can be irradiated to achieve a deep pink or red (Rubellite) color. This treatment appears to be permanent as well as undetectable.
- Oiling/Resin - This treatment is a form of fracture filling. The filler must have a refractive index that is close to the gem. This means that the light passes through the filler, it does so in a similar way that it passes through the gem material, making the fractures hard to see. Fillers include oils, resins, polymers and glass. Emeralds often contain natural fissures and are almost always treated in this way to disguise the fissures. Although this treatment is not always permanent, a gemstone can be re-oiled to restore it's treated look.
- Fracture Filling - Fracture filling has long been used in gemstones like Diamonds, Sapphires and Emeralds. This treatment fills fine cracks with a clear, glass-like substance. The cracks appear magically healed and most are invisible to the naked eye.
- Assembled - This is when a manufacturer joins two separate pieces of material together to form a single gem. The separate pieces used may be from natural stones or a combination of natural and man-made. A doublet is where two materials are fused together. A triplet is where three segments are joined, or where two materials plus a layer of colored cement are joined. Common assembled stones include, Garnet and Glass, Natural Sapphire and Synthetic Sapphire or Opals created by fusing a very thin layer of Opal to a backing and capped by a colorless dome.
- Dyeing - This treatment only works with porous gems like, Chalcedony, Lapis Lazuli or Jadeite. In a dyed gem, the material is dipped in a colored solution that soaks into the surface reaching fractures.
- Coating - This is a surface modification treatment, that alters the gems appearance. A colored coating is applied in order to deepen or add color. Colorless Quartz is often treated in this way to give it various colors.
- Diffusion - This treatment is a high temperature treatment applied to Corundum (Ruby and Sapphire). This is where colorless or lightly colored material is heated in a crucible containing iron, titanium or beryllium. The gems are heated almost to their melting point. If the crucible contains iron or titanium, the result is a thin layer of blue color that penetrates just below the surface. Diffusion in Corundum by using titanium oxide (Rutile), can create a Star Sappire. Because the treatment is so shallow, re-polishing the gemstone often will result in a partial or total removal of the color. On the other hand if the crucible contains beryllium, the color often will diffuse deep into or completely through the stone, permanently altering it's color. Sapphires treated with beryllium will often give vibrant yellows, oranges and red-orange colors.
- Composite - A few years ago large quantities of large, relatively clean Ruby material appeared on the market. The prices were very low and it was often sold as natural, heated Ruby. As it turned out this material was the result of low grade commercial Ruby being heated with leaded glass. Since the leaded glass has a similar refractive index to that of Ruby, the many cracks and fissures were masked. Also as more and more of this material came available, it appeared that there was more and more glass present and less Ruby. That is why it is now referred to as a composite and not a treated stone.
- Bleaching - This is where a chemical is used to bleach out the color of a gemstone. Many Cultured Pearls are bleached to remove dark spots or irregular color. Sometimes Jadeite is bleached to remove any brown modifiers, then it is treated with a polymer to fill the pits and voids caused by the acid treatment. After that it may also be dyed to add the desired color
In the distant past, people regarded natural gemstones as mysterious objects with magical powers. This began to change in the eighteenth century, as scientists began to unlock the physical properties behind the crystal structure of gems. By the end of the nineteenth century, many crystals were being grown in laboratories, that duplicated the the physical and chemical structures of many gemstones. Today, laboratory-grown gems are large enough and attractive enough to be cut and set into jewelry.
Synthetic Gems - The word synthetic actually comes from the Greek word for 'to put together'. Synthetics are grown in laboratories and are made up of the same chemical and crystal structure as their natural counterpart. Synthetics basically look the same as a natural gemstone, however on closer inspection, gemologists can detect evidence that separates them. Most of this evidence is obtained by looking at the gemstone under a microscope. There are a number of processes that are used to grow synthetics. Some are quite elaborate, resulting in more costly gems. One of the oldest processes dates back to the late 1800's and is called Flame Fusion. In this process, powered chemicals resembling the gem are dropped through a high temperature flame onto a rotating plate. The meted chemicals slowly cool, forming a gem crystal.
Imitation Gems - An imitation could be anything that is made to look like a natural counterpart. It differs from a synthetic in that it is not made up of the same crystal structure as the gemstone it is trying to imitate. Examples of imitations include, glass, plastic or synthetic gems that look like a corresponding gemstone. For years Synthetic Spinel of all colors has been used in birthstone jewelry as imitations for, Ruby, Sapphire, Aquamarine etc. An imitation could also be a lab created stone that has no counterpart in nature. A vivid example of this is Cubic Zirconia, which has been used for years in place of Diamond. Cubic Zirconia is not found in nature.
Due to the many synthetics as well as imitations on the market, it is highly advisable to know your jeweler and to purchase only from those who have the finest reputation.