Little is simple and straightforward in gemology. Every established principle has an exception. This also applies to gem ranking. There is no way to sort gems. Instead, there are several. Each has its own purpose, as well as exceptions that require close attention.
Precious and Semiprecious Gems
For centuries, people have used the terms “precious” and “semi-precious” to describe gemstones. You will still occasionally hear these terms today. However, this gem rating has so many exceptions that it has no real value. For example, diamonds are traditionally considered precious gemstones, but some sell for $100 a carat. You can see them (with enough magnification) as accent stones in cheap jewelry. Garnet gemstones, on the other hand, are traditionally considered semi-precious gemstones, but some sell for over $1,000 a carat, ten times the price of a low-quality diamond. Referring to a set of gem types as “precious” and all other gemstones as “semi-precious” can be misleading as it implies that gemstones have an inherently higher value. Professional gemologists no longer use these terms. If you find these descriptions, buyer beware.
Diamonds and Colored Stones
It is also common to find gems divided into two other categories: diamonds and colored stones. (Some merchants use the abbreviation “color” for colored stones). All gemstones, except diamonds, fall under the category of “colored stones”. Gem cutters and dealers use this gem classification system for two main reasons. Black Diamond: Properties, Carat, Colour, Sizes & Shapes
First, cutting diamonds requires special tools because these gemstones are harder than all others. With few exceptions, these tools are not suitable for cutting colored stones. So gem cutters need two different sets of tools to handle both types of stones.
Second, diamonds and colored stones are mined and distributed differently. Diamonds are one of the few gems with a consistent supply. However, the diamond industry leads the general public to believe that diamonds are incredibly rare. In fact, there are much rarer colored gemstones than diamonds.
Why does it happen? A near-monopoly controls the sales and marketing of diamonds. Those in charge are careful not to flood the market. Thus, diamonds retain their value. Meanwhile, excellent publicity has also convinced the public that diamonds are the main gemstones for engagement rings.
In this gem grading system, all diamonds, whether colorless or colored, are still considered diamonds. However, colorless diamonds and fancy colored diamonds – those that are any color other than light yellow – have different grading systems.
Likewise, all colored stones are considered colored stones, even if a specimen is colorless. For example, gemologists would still classify a colorless sapphire or topaz as a colored stone. Unlike diamonds, however, a colorless specimen is graded in the same way as a colored specimen.
Natural, synthetic, and imitation gemstones
Another way to classify gemstones is natural or synthetic.
What is the difference between natural and synthetic gemstones?
Natural stones, of course, form in nature. Synthetic stones are grown in laboratories. Natural and synthetic materials can share the same properties but still have considerable differences. The main difference is a rarity. A natural gem usually takes millions of years to form. Additionally, many people find that natural stones have aesthetic qualities not found in mass-produced materials. The value is another difference. As natural gemstones are rarer and take longer to form, they are more valuable than their synthetic counterparts. For this reason, distinguishing between natural and synthetic is an essential skill for gemologists.
What is the difference between synthetic and homo-created gemstones?
Stones created in laboratories can be further subdivided.
- Synthetic refers to materials that duplicate their natural counterparts. For example, synthetic emeralds, sapphires, and spinels share the physical and optical properties of natural varieties. In many cases, distinguishing natural and synthetic stones is quite challenging.
- Homocreate materials have no natural counterparts. This category includes laboratory-created gadolinium gallium garnet (GGG) and yttrium aluminum garnet (YAG).
For a long time, gemologists considered cubic zirconia (CZ) a homocreated gem. However, tiny CZcrystals not large enough to be used as gemstones have been found in nature as the mineral baddeleyite. This discovery means that CZ is a synthetic stone rather than a homocreate.
What is imitation jewelry?
A simulant or imitation gem is any material presented as a “double” of another gem. For example, a natural white topaz can be sold as an imitation diamond. On the other hand, it can also be sold as real topaz. A CZ described as cubic zirconia in a jewelry ad is not an imitation. In contrast, a CZ represented as a “look-alike” diamond is an imitation. Amethyst Gemstones Guide: Price, Quality & Value
Many vendors often sell imitation gemstones, typically for much less than the imitation gemstones would cost. As long as consumers are aware that they are buying “fake pearls” or “fake diamonds,” for example, there is nothing unethical about it. However, selling imitation jewelry as real is unethical.
Organic and inorganic gemstones
Another approach to classifying gemstones is to separate gemstones into organic and inorganic. Organic refers to gemstones whose formation involves living organisms. Amber, for example, started out as tree sap. Various mollusks create pearls. Therefore, these gem materials are classified as organic.
The term inorganic encompasses everything else. So, everything in the mineral world falls into the inorganic classification.
However, not all inorganic gemstones are minerals. In the United States, a gem can only be classified as a mineral if it was created geologically on Earth. Thus, laboratory-created stones, while having inorganic origins and the same properties as their natural counterparts, cannot legally be described as minerals. For gem classification purposes, these lab-created gemstones have the same properties as their natural mineral counterparts. However, ads cannot describe them as minerals.
Crystalline and Amorphous Materials
Differentiating between crystalline and amorphous materials is another way of classifying gemstones. The term “crystalline” refers to minerals composed of a repeated arrangement of atoms. The term “amorphous” refers to materials that do not consist of a repeated arrangement of atoms. While some materials may look like crystals to the naked eye, only their atomic structure matters in making this distinction. For example, a glass cube may look like a crystal, but its atomic structure remains noncrystalline. Glass, both natural and manufactured, is an amorphous material. Not all gemstones are crystal clear. Amber and opal, as well as glass, are good examples of amorphous gem materials. Amorphous materials can have organic or inorganic origins. Examples of amorphous organic materials include amber and ivory. Inorganic amorphous gemstones include opal. Garnet (Power Stone) Meaning, Healing Properties, Uses, & Benefits
The term “aggregate” applies to groups of small gems that form together. Aggregates form when the requirements necessary for yolk formation – such as certain chemicals, heat, pressure, and space – are not present for the time required.
Although an aggregate may appear amorphous, it internally consists of thousands of microscopic crystals. The most common example of aggregate minerals is the chalcedony family, which includes agate and jasper. These members of the quartz family share many common characteristics. Therefore, these aggregates may have the same specific gravity and refractive index as a whole quartz crystal, but very different appearances.
While crystals and amorphous materials have a single main ingredient, a mixture of minerals comprises rocks. Although not a gem material, granite is one of the most common and well-known rocks. If you look closely at a sample, you’ll see black, white, and gray chunks all joined together. You won’t see many rocks in gemology. Lapis lazuli is perhaps the most well-known rock commonly found in the gem world.
Classification of minerals and gems
Now we’re getting into the heart of gem ranking. The vast majority of gemstones are minerals. Both chemical composition and molecular structure define mineral species.
Chemical makeup?? refers to the atoms contained in the mineral. Diamond, for example, has the simplest chemical composition. Carbon (C) is the only element present. Corundum is composed of just two elements, aluminum (Al) and oxygen (O), expressed as the formula Al 2 O 3. This means that a molecule of corundum contains two atoms of aluminum and three atoms of oxygen. The chemistry of other gems gets more complicated. For example, the chemistry of tourmaline ?? can be expressed as Na (Li, Al) 3 Al 6 B 3 Si 6 O 27 (OH) 3 (OH, F). Discover why diamond is so valuable and precious
Molecular structure refers to how molecules bond to each other. While you can’t see individual atoms, you can see the results of how they bond together in whole crystals. Diamonds form crystals that look like two pyramids attached to their bases. Quartz forms elongated crystals with six sides. These are the results of their molecular structures. For example, imagine you have two sets of parts. The four-sided tiles will form a kind of design. The six-sided tiles will form an entirely different set of designs. The two styles cannot fit together. Each set constitutes a different crystal system.
Example: Diamond and Graphite
Used in pencil lead, lead is very soft and black. The hardest substance in nature, diamond (generally) is colorless. Graphite and diamond have the same chemical composition: pure carbon. So what explains the difference in appearance and hardness? Their different arrangements of carbon atoms. The carbon atoms in diamonds are arranged in a tetrahedral pattern, where each carbon atom is bonded to four other carbon atoms. This structure is incredibly stable, which explains how hard a diamond is. On the other hand, the carbon atoms in graphite are arranged in a “chicken coop” pattern. This structure is less stable. This makes the graphite soft.
When minerals share a chemical composition but have different molecular structures, their molecular structures define the type of mineral. Some minerals share a molecular structure but have different chemistry. In these cases, the chemical compositions define the type of mineral.
Mineral species and varieties
Throughout this article, we have discussed pure minerals. In nature, minerals usually have impurities present in very small amounts, usually 3% or less of the crystal by weight. These impurities do not change the primary chemistry. Therefore, the name of the mineral, or species, does not change. However, they change some of the mineral’s characteristics, so we use a subclassification called variety. Changes in characteristics, such as color, can have a considerable effect on the value of a gem mineral.
Colors and varieties
Many pure minerals are colorless. Impurities give them color. For example, pure corundum is colorless. Add some chrome and we call it ruby. Add some titanium and iron and you have a blue sapphire. Pure beryl is also colorless. Add a touch of chrome and we have an emerald. Add some iron and you have aquamarine. Just a small amount of impurities can make a mineral exceptionally valuable!
Corundum and beryl are called mineral species. Their colored versions are “varieties”. Another very common species is quartz, with colored varieties such as amethyst, citrine, and smoky quartz.
There are always one or two exceptions. Here we go. Not all minerals are colorless in their pure state. Garnet is one of the most obvious examples. In addition, there are several species of garnets, as well as varieties. All grenades share the same structure and many similarities in their chemical composition. However, they have variations in chemistry. Each of these variations amounts to a new species of garnet.
Here is an illustration that is not scientifically accurate but still helps to explain how grenades vary. Look at your hand and pretend it’s a model of a grenade molecule. All grenades have the same structure, shape of your hand, and pretty much the same chemistry. The last knuckles of your fingers represent separate atoms. Although most atoms remain the same, different atoms can reside in these joints. If you change the atoms (chemistry), you change the species. That’s the rule. However, you can see that your hand shape has not changed or displayed any of the other basic characteristics. Consequently, the different species are still garnets. 10 best cell phone cases of 2022
Series and Blends
Common red grenades are almandine or pyrope grenades. Both garnet species are deep red. However, each has a slightly different chemical composition. The purest almandine garnet ever found contains about 20% pyrope. The purest pyrope also contains about 20% almandine. Each of these gems also contains a small number of other garnet species. When gemologists need to name a grenade, they call it the major component. As you can see, this distinction is not always clear.
Consider this: if the purest pyrope garnet ever found is only 80% pyrope, then there are many more specimens that are closer to being only 50% pyrope. Gemologists describe most garnets simply as an almandine-pyrope mixture.
Some garnet blends take on a distinct set of characteristics. For example, a rhodolite is approximately 70% pyrope and 30% almandine. What makes it distinctive is its purple coloring, as its two main components are red. This quality is distinctive enough that rhodolite is considered a garnet variety.
Grenades never occur in their pure state, but always in combination with each other. For example, most gem-grade grenades are in the almandine-pyrope-spessartite series. Almandine, pyrope, and spessartite are individual species of garnet and are always found together. The species that make up the majority give the gem its name. This type of mixing, where the species of a specific gem are always found together, is called a solid-state series.
Do other minerals form series?
Garnets are not the only minerals that form a series. Feldspars and spinels also form in a series of solid states, like garnets.
Mineral groups and gem classification
Scientists also classify minerals into groups. While more important to mineralogists than gemologists, learn the terminology. The fields overlap and terms occasionally appear in gemological texts.
Not all species in a group can be considered quality materials for gemstones. For example, facets usually cut only the three previously mentioned garnet species, plus andradite, grossular, hydrogrossular, and uvarovite (as well as their combinations and varieties). Non-gem-quality grenades include goldmanite, kimzeyite, knorringite, schoolmate, and yamatoite.
A similar situation exists for the tourmaline and feldspar groups. Only a few of its members are often used as jewelry.
Mineral classes and gem classification
Shared chemistry can also categorize minerals. For example, all minerals that contain silica are grouped together as silicates. While not important for all gemologists, gem cutters may find this information useful. For example, when lapidaries are cutting a gem for the first time, they must determine the best polishing compound. Identifying the gem’s class makes it reasonable to start with compounds that work for other gems in that grouping.