There are any number of ways by which gems can be classified. The remainder of this presentation describes several of the most common ways:
· PRECIOUS OR SEMIPRECIOUS: (HISTORICAL VIEW OF VALUE)
· FACETED OR CABOCHON: (CUTTING STYLE)
· NATURAL OR SYNTHETIC: (ORIGIN)
· ENHANCED OR UNENHANCED: (TREATMENT STATUS)
· SIMULANT OR FAKE: ( HOW REPRESENTED)
· COLORED STONE OR DIAMOND: (GEM INDUSTRY VIEW)
· JEWELRY OR COLLECTOR GEM: (WHO WILL BE THE END USER)
Classified By Historical View of Value: "Precious or Semiprecious".
These terms were routinely used (until about the 1980's) to separate diamond, ruby, sapphire, emerald and sometimes pearl, from all other gem species such as tourmaline, jasper and amber. Most gemologists no longer use these words and consider them out-moded. Why?
The term, "precious", implies rarity and high value, but, in reality, the individual specimens of each gem species and variety exist within in a full spectrum of rarity, and of value, from very low to very high. Some pieces of "semiprecious" gems are rarer, and more valuable than some individual specimens of "precious" gems.
[Precious? ruby (in zoisite) worth about $1-$2/ct -- semiprecious?
So what should we call them then? Simple: ----> gemstones or gems<-----. These terms will cover them all, regardless of where a given piece lies within the continuum of rarity, beauty, and value for its species.
So seriously is this idea taken that the "Code of Ethics and Principles of Fair Business Practices" of the American Gem Trade Association (a colored stone trade organization) instructs members to "avoid the use of the term "semiprecious"in describing gemstones", and they have purgedthat term from all their publications.
Classified By Cutting Style: Faceted or cabochon cut:
They are the two most common ways in which gems are fashioned.
Faceted stones are usually cut from transparent rough of relatively high clarity. They are fashioned with a top (crown) and a bottom (pavilion) that have intersecting flat planes called facets, on their surfaces. These facets have shapes that are generally triangular, kite shaped or rectangular.
Cabochon ['kæbəʃɔn] cutting is most often used for translucent and opaque gems and such pieces generally have a flat bottom and a smoothly curved top called a dome.
[Faceted peridot, cabochon cut lapis lazuli]
The parts of a faceted gem:,
Girdle: The girdle is the divider between the top and bottom of the gem. It defines the face-up outline, and the maximum dimensions of a faceted gem. In well proportioned stones, it usually comprises about 2% of the total depth of the gem.
Crown: The top, the part of the gem above the girdle is known as its crown. In a well proportioned stone it makes up 1/4 to 1/3 of the total depth of the gem.
Table: The largest, usually central, facet on the crown of a faceted gem is the table. Generally, it makes up between 40 - 70% of the crown diameter.
Pavilion: The pavilion is the bottom, the part of the gem below the girdle. In a well proportioned gem, it usually accounts for 2/3 to 3/4 of the total depth of the gem.
[Pavilion view diagrams of round and emerald cut faceted gems]
Culet/Keel龙骨线: The tip or line at the bottom of the pavilion on a faceted stone where the pavilion facets meet.
[Culet on a square cut stone, keel on an emerald cut stone]
Classified By Origin: Natural or Synthetic
Natural: A natural gem is one produced entirely by geologic and/or biological processes without any human input or assistance.
Synthetic: laboratory grown, manufactured, or "cultured" by human intervention.
A synthetic can be a copy of a natural mineral such as corundum, amethyst, or pearl, or it can be a unique material not found in nature like YAG (yttrium aluminum garnet) or cubic zirconia. In addition to the use of synthetics as gem substitutes, they are also made for industrial, electronic, and research purposes. Examples include synthetic diamonds used as abrasives, and synthetic rubies and garnets used in lasers.
Classified By Treatment: Unenhanced or Enhanced:
Unenhanced: means (except for cleaning and/or fashioning into a useable gem) that the material is as it was yielded from Nature. The color, transparency, hardness, or optical phenomena have not been changed by man.
**Among the general public "natural" is often misunderstood to mean unenhanced--> but in the reality of the gem trade, the term natural, does NOT equal the term unenhanced. When gems are formally described and graded, the origin (natural-vs-synthetic) is a totally separate factor from the treatment status (unenhanced-vs-enhanced)** Unfortunately, many sellers know that by using the word "natural" in the description of a gem (which may be true) their buyers will assume that the gem is untreated (which is very likely not to be true).
Enhanced: an enhanced gem has received some type of treatment to change its characteristics: Ex. irradiation, heating, dyeing, oiling, laser drilling, etc. There are numerous treatments, some of which are routine, have little effect on value, and are considered acceptable as long as they are disclosed to the buyer, and others which are considered extreme and which dramatically alter the value of the gem. A treatment may increase, decrease or have no effect on the durability of a gem. (In a later web lecture, gem enhancement will be covered in detail.) The examples below are of some of the most common, well accepted treatments that have minimal effect on gem value.
[Routinely enhanced gemstones: black onyx (dyed to change color), emerald (oiled to increase clarity), sapphire (heated to change color), blue topaz (irradiated, then heated to change color)]
Classified By Intended Use: Simulant or Fake
Simulant: a material, either natural or synthetic, which is being used to imitate another material. Simulants look like what they imitate, but they may or may not share its chemical, physical and optical properties. Not all simulants are synthetics! These mimics are correctly termed either "simulant", "imitation", or "faux".
Ex. synthetic ruby can be used to simulate a natural ruby, but it is also possible for natural red spinel to be used to simulate a natural ruby.
Fake: any material which is represented as something it is not. The fake can be of man-made, or natural origin. **Whether something is a fake or not, is simply a matter of "truth in advertising".**
Not Fakes:
*A synthetic ruby offered as a synthetic ruby.
*Man-made red glass offered as a "faux" ruby.
* A cubic zirconia offered as a "diamond simulant".
Fakes:
* A natural red garnet offered as a ruby.
*A man made Moissaniteoffered as a diamond.
*An enhanced colored diamond offered as an unenhanced colored diamond.
Simple test: if the material is represented accurately, it is not a fake, if it is represented inaccurately it is a fake, regardless of whether it is natural or man-made!
Classified By Industry Terms: Colored Stones or Diamonds:
Gemologists put all colored stones together into one category and all diamonds into a separate one--> regardless of their color! The reasons for this is that there are great differences which exist in the systems for fashioning, grading and marketing these two categories of gems. This distinction doesn't divide cleanly, however, between all stones that show color, and all that are diamonds.
Some gems which are classified as "colored stones" are, in fact, colorless. Examples would be white sapphire, white beryl, phenakite硅铍石 and rock crystal quartz. Some diamonds have color, in fact, they are referred to as "fancy" diamonds, amongst which we find the green, pink, blue, yellow, orange, brown, red and black diamonds.
Classification is simple: is it a diamond?, No -- then it's a colored stone (regardless of its color or lack of it!)
[Colored stones: pink sapphires, faceted phenakite, rough rock crystal quartz]
[Not colored stones: black and white diamonds in a pendant, brown diamond rough, cut yellow diamond: Yellow diamond image courtesy of www.thaiambergems.com]
**Check the text: Pages 38 and 39 of the Hall book show 23 examples of transparent colorless "colored stones" and page 41 shows a number of others that are translucent to opaque white.
Classified by who will be the "End User": "jewelry" gems or "collector" gems:
This distinction is not as clear cut as some of the others. Although there are over 3000 species of minerals, of which only 100 - 150 have the characteristics that we associate with gems, and of these, only about 50 species make up a regular part of the jewelry marketplace. In reality, though, the properties of jewelry and collector stones overlap and grade into one another.
In general, a jewelry gem is one that is both durable enough to be used for most jewelry applications, and common enough to be found in the marketplace in at least moderate amounts. Aquamarine is a good example of a jewelry gem. It is both durable enough and common enough to be readily used, and is widely found in the jewelry marketplace.
[Aquamarine, a jewelry gem]
A collector gem is one that is either not durable enough to be used in jewelry, or so rare that it is not found within the common market channels for jewelry.
Transparent rhodocrostite 菱锰矿is an example of a collector gem which, is not durable enough to be set and worn in jewelry, although it is abundant enough to have a place in the jewelry market if it were useable.
Clinohumite斜硅镁石is an example of a collector gem which is quite durable enough for most jewelry uses, but so rare that only a few collectors are able to obtain specimens, so it is not found within the normal gem and jewelry channels.
[ Classified as collector gems for different reasons: gem rhodocrosite, clinohumite]
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Answers to the thought exercise for this lesson. (If you don't understand why these are the correct answers, then it's a good time to email me and ask!)
1): Each of these items has some of the requisites for being a gem, but lacks at least one of the crucial defining properties. A butterfly wing is natural, beautiful, and may be rare, but it is not durable. Industrial grade diamond is natural and durable, and is rare, but it is not beautiful (which is why it is used only industrially), quartz beach sand is natural, durable and if you were to examine it under a microscope you'd see it is, in fact, beautiful, but it is not rare. A laboratory grown ruby is beautiful and durable, but it is neither rare nor natural.