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GemSelect Newsletter - The Science of Gemstone Classification

Gemstone Groups
Gemstones by Type
The Science of Gemstone Classification

Have you ever wondered how gemstones are classified? Well, according to the science of mineralogy, gemstone classification begins by distinguishing various gemstone groups based on their crystal structures and associated chemical composition. Gemstone groups are then subdivided into separate types and lastly, varieties. The process seems simple enough, but in actuality, most average consumers are unaware that groups, types and varieties are actually different gem classification levels. The science of gemstone classification was defined by former GIA graduates, Cornelius Hurlbut and Robert Kammerling, and remarkably, their system is still used by gemologists today.

Gemstone Groups

When two or more gem types have a similar chemical composition, crystal structure or physical qualities, they are defined as a group. There are 16 gemstone mineral groups, consisting of beryl, chrysoberyl, corundum, diamond, feldspar, garnet, jade, lapis lazuli, opal, peridot, quartz, spinel, topaz, tourmaline, turquoise and zircon. Interestingly, there are several gem "groups" that are actually stand-alone gem types, such as tourmaline, zircon, topaz and spinel. However, each of these types has multiple variations available, therefore justifying group classification status.

Gemstone Types

Gemstone types have varying chemical compositions, as well as crystal structures. There are approximately 130 or more different gemstone types on the market today, with new discoveries being added. Chemical compositions can range widely, from complex mixes of various compounds to simple, single chemical elements, like diamond which is composed of only carbon. Although diamond may have a simple composition, its crystal structure is quite complex. Crystal structures can also vary from simple single structures to immensely complicated clusters of microscopic crystal. Some types may even possess a non-crystalline structure such as opal, referred to as an amorphous structure. Inorganic gems are classified based on chemical composition, as well as crystalline structure similarities, whereas organic types are classified by chemical composition only. Examples of organic species include pearl, coral, amber and ivory.

Gemstone Varieties

Gem varieties are subdivided and branched from gem types. A perfect example would be corundum, which is a group. Blue corundum is known as sapphire and red corundum is ruby, both of which are gemstone types. Sapphires that possess asterism become a variety of sapphire - star sapphire. Varietal classifications are based on optical qualities including color, optical phenomena, color distribution and transparency.

Corundum Gemstones
Corundum Gemstones
Color:

Color is produced through the absorption and transmission of light at certain frequencies. The wavelengths of specific electromagnetic vibration determine the visible color perceived by the human eye. Colors are described using a combination of varied hues, tones and intensity levels. Some examples of gem varieties classified by color include citrine, the yellow-gold variety of quartz; and amethyst, which is the violet variety of quartz.

Optical Phenomena:

Optical Phenomena: The most common of optical phenomena is iridescence. Iridescence includes traits such as orient, play of color and labradorescence. Fire agate is an example of a gem variety classified by optical phenomena. Fire agate is simply brown agate with iridescence, caused by layers of plate-like crystals of iron oxide, or limonite. As a result of the reflection of light from thin layers of limonite, red, gold, green and blue-violet colors are displayed. Some gem varieties are distinguished by chatoyancy (the cat's eye effect), such as cat's eye tourmaline or cat's eye quartz. Other types of optical phenomena include adularescence, aventurescence, asterism (the star effect) and color change.

Color Distribution:

Color Distribution: Color distribution refers to the distribution of solid colors and patterns on a gemstone. Some gemstones distribute colors and patterns so unique that a separate varietal name is needed in order to properly classify them. One example is onyx, which is essentially agate formed with parallel rather than curved band patterns.

Transparency:

In the gem trade, transparency refers to a gem's ability to transmit light, and gems are often classified by their transparency. Gemstones can be transparent, translucent or opaque. Colorless quartz is an example of a transparent variety, whereas agate and moonstone are typically translucent. Both jasper and tiger's eye are considered to be opaque, because no light is passed through these varieties.

Organics & Synthetics

Organic gems are naturally occurring gems that are the result of biological processes. This is a small group of gems including jet (bituminous coal), coral, ivory, pearl and amber.

Synthetic gems were created in a laboratory setting. Synthetic gems have "natural" composition, but they were not naturally occurring. When compared to their naturally occurring counterparts, synthetic gemstones have identical chemical composition and crystal structure, including specific gravity and other various optical qualities. It should be noted however, that not all lab-created gems are considered to be synthetic. This is because some lab-grown gems utilize unnatural ingredients, such as lab-grown opal, which is composed of silica (70-80%) and bonding agents (20-30%).

Synthetics are very often confused with "simulated" gems, but simulated gems are simply imitations possessing only optical similarities. A diamond simulant known as cubic zirconia is a perfect example of a simulated gem; CZ appears to be the same as natural diamond, but the chemical and crystal structures are worlds apart.

Customer Questions

Every month we answer questions of general interest from our customers. Please feel free to send your questions or suggestions to our support team at help@gemselect.com!

Question
How do I order more than one gemstone at a time? I want to combine items, so I pay only a flat $6.99. Is there a limit to how many items I can combine per transaction?
Answer
The great news is that there is no quantity limit for our flat rate shipping. For one low flat rate of $6.99, you can combine unlimited items to your order and we'll ship them to just about anywhere in the world. It's very easy to add multiple items to your shopping cart and we encourage everyone to take advantage of our combined shipping. In fact, we've made it 100% effortless. On our product gallery pages, simply select the (2nd) middle button - "add to cart", rather than the "buy it now" option. Now you can continue shopping until you're ready to check-out. You can find the "add to cart" button just below the "click for details" button.
Question
After I placed an order on your website, I left a comment on your BizRate feedback survey, but I checked your customer review page and my feedback wasn't there. Does the GemSelect team moderate these comments, and if not, how can I share my review?
Answer
We can confirm that that GemSelect does not moderate any reviews left for us on Bizrate.com. In fact, this is exactly why we chose to use the BizRate (ShopZilla) Merchant Review program. Only those who have placed real orders through our website are offered an opportunity to share their experiences with others.
BizRate will only show comments from those who complete a survey after receiving an order (as opposed to the time of check-out). The reason for this, is that BizRate wants to ensure you've actually received your order so that you can also comment on other important aspects of online shopping, including on-time delivery, product expectations and overall satisfaction.
If you'd like to share your GemSelect experience, please reply to the BizRate feedback email with the subject line: "Shipping verification reminder - response requested". This email is typically sent a few days after the expected delivery date entered during the initial feedback survey.

We hope you found this article interesting. If you have any comments or suggestions just drop us a line. We really do read every single email we receive and each email gets a personal response.

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Happy Gem Hunting!
Your friends at GemSelect

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サイズとウェイト

Gems are always measured in Millimeter (mm)

Dimensions are given as;
length x width x depth,
except for round stones which are;
diameter x depth

Select gems by size, not by weight!
Gem varieties vary in density, so carat weight is not a good indication of size

Note: 1ct = 0.2g

Size Comparison Chart