by Donald Clark, CSM IMG
A Guide to Gem Classification
There is little that is simple and straightforward in gemology. For every established principle or example, there is always an exception. This applies to gem classification as well. There is not one way to classify gems, rather there are several ways, each of which has its own purpose and its own exceptions that warrant close attention.
Precious and Semiprecious
A couple of centuries ago, the terms precious and semiprecious gems came into common use. There are so many exceptions to this classification that it no longer has any value. For example, diamonds have always been considered precious gems, yet there are diamonds that sell for $100 a carat. You can see them, with sufficient magnification, as accent stones on inexpensive jewelry. On the other hand there are garnets that sell in excess of $1,000 a carat. Garnets have traditionally been considered semiprecious gems, but some of them are worth more than ten times a low quality diamond.
These terms are still used occasionally, but they are frowned upon. In fact, the U.S. Federal Trade Commission, which governs marketing to consumers, has considered making it illegal to use them because they can be deceptive. Since these terms are no longer used, it is best to eliminate them from your vocabulary, especially if you want to appear professional and well educated.
Diamonds and Colored Stones
Another way to classify gems is by categorizing them as either diamonds or colored stones (some trade dealers use the shorthand color). There are two main reasons that diamonds are separated from colored stones using this classification system. First, they are harder than all other gemstones, which means special tools are required to cut them. These special tools are not suitable for colored gems because they are not as hard, so gem cutters need two different sets of tools to be able to handle both types of stones. There are some exceptions to this, but for the most part, colored gems can not be cut and polished with the same tools as diamonds.
The second difference between diamonds and colored stones is how they are mined and distributed. Diamonds are one of the few gemstones in the world with a consistent supply, despite the fact that the diamond industry leads the general public to believe that they are incredibly rare. This is possible because the marketing and sale of diamonds had been monopolized, and those in charge are careful not to flood the market so the diamond maintains its value. Those who control the diamond industry have done an incredible job convincing the public that there are diamonds and then all other gems, despite the fact that there are some colored gemstones that are much rarer than diamonds.
Natural and Man-Made
Yet another way of classifying gemstones is using natural or man made classifications. There are a couple of terms commonly used for gem material that is created in a laboratory. Synthetic refers to materials that duplicate their natural counterparts. It is quite common to see synthetic material for emeralds, sapphires and spinel.
Homocreate materials have no counterpart in nature. This category includes the synthetic garnets, GGG and YAG. For a long time, cubic zirconium was thought to be a synthetic gemstone; however, tiny crystals have been found in nature. Though these crystals were not large enough to be used as gems, it proves that cubic zirconium is actually made from synthetic materials rather than homocreate materials.
While natural and man made materials can share the same physical and optical properties, there are still considerable differences. The main difference is rarity. A natural gem takes quite a bit of time to form and is usually millions of years old. Plus, many feel they have aesthetic qualities not found in mass-produced materials. Another difference is value. Because natural gemstones are rarer and take longer to form, they are more valuable than their man made counterparts. It is for this reason that being able to distinguish between the two is essential.
It is also worth pointing out the definition of an imitation. Anything that is posing as something else is an imitation. For example, a white topaz posing as a diamond is an imitation. A CZ, described as a Cubic Zirconium (for example, in a jewelry ad), is not an imitation. By contrast, a CZ that is represented to be a diamond is an imitation.
Organics and Inorganics
Another approach is to separate gems into organics and inorganics. Organic gems are those whose creation is associated with living organisms. Amber, for example, begins life as tree sap and pearls are created inside oysters. Hence, they are classified as organic materials.
The term inorganic covers everything else. So, everything in the mineral world falls into the inorganic classification. There is one notable exception that arises because of politics here in the U.S. In order for a gemstone to be classified as a mineral in this country, it has to have been created in the earth. Thus lab-created stones, even though they may have the same chemical make up, physical, and optical properties of their natural counterparts, cannot legally be described as minerals in the U.S. For classification purposes these lab-created gems have the same properties as their natural mineral counterparts; however, they cannot legally be described as minerals, especially when advertising.
Crystalline and Amorphous Materials
Differentiating between crystalline and amorphous materials is another way gemstones are classified. The term crystalline refers to minerals that are comprised of a repeating pattern of crystals, while the term amorphous is used to describe minerals that have no set form or shape. While many gems are crystalline in structure, not all gems are crystalline. Amber and opal are good examples of amorphous materials. Glass, both natural and man made, is also an amorphous material. Amorphous materials can be both organic and inorganic. Examples of organic amorphous materials include amber and ivory, and opal is an example of an inorganic amorphous gem.
The term aggregate applies to collections of small gems that form together. Aggregates form when the requirements needed for crystal formation, such as the presence of certain chemicals, heat, pressure, time, and space, are not present for the necessary amount of time. For example, if the material cools off too quickly or there is not enough space for the crystals to grow, an aggregate forms instead of one full crystal.
An aggregate will look much like an amorphous material, but internally it is composed of thousands of microscopic crystals. The most common example of aggregate minerals is the chalcedony family, which contains agates and jaspers. These are all members of the quartz family, so they share many common characteristics. These aggregates will have the same density and refractive index as a whole crystal of quartz, but considerably different appearance.
Rocks are comprised of a mixture of minerals, whereas crystals and amorphous materials have a single main ingredient. While not a gem material, granite is one of the most common and best known rocks. If you look at it carefully, you will see black, white and gray bits all bound together in a single material. Though you do not see too many rocks in gemology, there are a few, including lapis lazuli, the most well known rock in the gem world.
Now we are getting into the heart of how gems are classified. The vast majority of gems are minerals. Mineral species are defined by a combination of their chemical makeup and their molecular structure.
Chemical makeup refers to the atoms contained within the mineral. Diamond, for example, has the simplest chemical makeup with carbon (C) being the only element present. Corundum is composed of just two elements, aluminum and oxygen. Its chemistry is expressed as Al2O3, meaning there are two aluminum atoms and three oxygen atoms in a molecule of corundum. The chemistry of other gems can get a lot more complicated. For example, tourmaline’s chemistry is expressed as Na(Li,Al)3Al6B3Si6O27(OH)3(OH,F).
Molecular structure refers to how the molecules attach to each other. While you can’t see the individual atoms, you can see the result of how they attach to each other in whole crystals. Diamonds form crystals that look like two pyramids attached at their bases. Quartz forms elongated crystals with six sides. This is a result of the molecular structure. It’s as if you had two sets of tiles to work with. Those with four sides will form one kind of design, others with six sides form an entirely different set of designs. The two styles cannot fit together because six sided tiles and four sided tiles each make up different systems.
These two elements, chemical makeup and molecular structure, must be taken together when defining a mineral. The best example is comparing diamond and graphite. Graphite is used in pencil leads; it is very soft and black. Diamonds are the hardest substance in nature and colorless. Both diamond and graphite have the same chemical makeup, being comprised of pure carbon, so what accounts for the difference in appearance and hardness? The way the carbon atoms are arranged. The carbon atoms in diamonds are arranged in a tetrahedron pattern where each carbon atom is bonded to four other carbon atoms. This structure is incredibly stable, which accounts for a diamond’s hardness. The carbon atoms in graphite, on the other hand, are arranged in a pattern that resembles chicken wire. This arrangement is less stable, which is why graphite is softer than diamond.
While diamond and graphite represent an example where two minerals share the same chemical makeup, yet have different molecular structures and consequently different names, there are also minerals that have the same molecular structure, but different chemistry. With these minerals it is the chemical makeup that defines the mineral type.
Species and Varieties
It is important to note that throughout this article we are discussing pure minerals. In nature it is common for a mineral to have impurities. These are present in very tiny amounts, usually 3% or less of the crystal by weight. These impurities do not change the primary chemistry, so the mineral name, or species, does not change. They do, however, change some of the mineral’s characteristics, so we use a sub-classification called a variety. While impurities do not make a significant difference in the chemistry of a mineral, they can make a significant difference in a mineral’s appearance, which can have a considerable effect on its value.
Many pure minerals are colorless; it is the impurities that give them color. An excellent example is corundum. In its pure form it is completely colorless. Add a bit of chromium and we call it a ruby; add a bit of titanium and iron and it becomes a blue sapphire. Pure beryl is also colorless. Add a touch of chromium and you have an emerald; add a bit of iron and you get an aquamarine. Just a tiny bit of these impurities and a mineral suddenly becomes exceptionally valuable!
Corundum and beryl are called mineral species. Their colored versions are varieties. Another very common species is quartz, which has the varieties of amethyst, citrine, and smoky quartz.
Now there always has to be an exception or two, so here we go. Not all minerals are colorless in their pure state. Garnet is one of the most obvious examples. Also, there are several species of garnets as well as varieties. Garnets all share the same structure and a lot of similarities in their chemical makeup; however, they do have variations in chemistry. Each variation of chemistry equates to a new species of garnet.
The following example is not up to scientific standards of accuracy, but it will help to illustrate how garnets vary. Look at your hand and consider it to be a model of a garnet molecule. All garnets will have the same structure, the shape of your hand, and pretty much the same chemistry. The last joints of your fingers represent separate atoms. While most of the atoms remain the same, different atoms can reside in those places. If you change the atoms (the chemistry), you change the species — that’s the rule. However, you can see that the shape of your hand has not changed nor have any of the other basic characteristics. Hence they are still garnets.
Common red garnets are either almandine or pyrope garnets. Both of these garnet species are deep red; however, each has a slightly different chemical makeup. That being said, the purest almandine garnet ever found contains 20% pyrope and the purest pyrope contains 20% almandine. Each of these gems also contains a minute amount of other garnet species. When a gemologist needs to put a name on a garnet, they will call it by the component that is in the majority. As you can see, this is not always a clear distinction. If the purest pyrope garnet ever found is only 80% pyrope, then there are a lot more that are closer to being only 50% pyrope. For most garnets, it is best to describe them simply as almandine/pyrope.
There are some garnet blends that take on a distinct set of characteristics. A good example is a rhodolite garnet. A rhodolite is approximately 70% pyrope and 30% almandine. What makes it distinctive is its purple coloring. Remember the two major components are red, hence the purple is distinctive. This quality is distinct enough that rhodolite is considered a variety of garnet.
Series and Blends
As mentioned above, garnets are never found in their pure state, but always in combination with each other. For example, most of our gem-grade garnets are in the almandine—pyrope—spessartite series. Almandine, pyrope, and spessartite are individual species of garnet and they are always found together. The element that makes up the majority is the one whose name is given to the gem.
This kind of blend, where species of a specific gem are always found together, is called a solid state series. The feldspar minerals also form in a series like this.
Minerals are also classed as groups. This is more important to the mineralogist than the gemologist, but it helps to know the terminology, since the two fields overlap and the terms show up in gemological text books from time to time.
The garnet group would contain the three species mentioned above, almandine, pyrope, and spessartite, plus hydrogrossular, kimzeyite, goldmanite, schorlomite, knorringite, yamatoite, andradite, and uvarovite. Of these later additions, only the last two are gem material.
A similar situation exists for the tourmaline and feldspar groups. They have several members, but only a few are used as gems.
Minerals are also categorized by common chemistry. For example, all minerals that contain silica will be grouped as silicates. This kind of grouping is not important to all gemologists; however, it is important to mineralogists, chemical gemologists, and occasionally to the gem cutter. For example, if a lapidary is about to cut a gem for the first time, the best polishing compound is a mystery. If he knows what group the gem in question belongs to, it would be reasonable to start with compounds that work for other gems in that group.