Gemology  
     

 

 

                                                                                                            Gemology 
                                                    
By. Saurabh seth 
                                                                               www.indiangemstonelab.com

    OPTICAL PROPERITES OF GEMS

 Optical properties are those which are related to the behavior of light, on, or in, a gemstone.

Some of these can be seen, and even quantified, with the naked eye alone. Three such characteristics are: luster, transparency, and color. The study of these factors, and their use in gem identification gem testing and evaluation, is sometimes called optical gemology.

Other characteristics are revealed, or measured, only through the use of special instruments. Some of these include: refractive index, optical character, birefringence, pleochroism, dispersion, reaction to ultraviolet light and selective absorption. When these properties of gems are analyzed and measured, one is engaging in laboratory gemology.

 

Luster

 

The luster of a gemstone is comprised of the quantity and quality of the light reflected from its surface. There is an inherent, potential luster possible for each species and variety of gemstone. The actual luster, on any individual piece, however; may be less than this, due to the skill level of the lapidary or facetor, the presence of inclusions, or various chemical or physical changes, such as oxidation or abrasion, that can affect the surface.

The names, which have been given to the various lusters seen in gems, are derived from their resemblance to familiar surfaces. (The prefix sub- indicates "just less than".) Some lusters are so embodied by a particular stone, that its appearance is named for that stone, as in the case of adamantine luster (adamas = Greek for diamond), and pearly luster. Looking through either of your textbooks at the descriptions of the various gems will convince you that a substantial majority of gems have a glass-like or "vitreous" luster.

Look at the picture of the fire agates below and compare what you see on their surfaces to that which you'd see on a freshly washed and dried drinking glass--> keeping that image in mind should help greatly in estimating the luster of a gem.

 

 

 

[Pyrite (in shist): metallic; diamond: adamantine (like diamond); zircon: subadamantine; fire agate: vitreous (like glass)]

 

 

 

 

 

 

 

[Fluorite: subvitreous; nephrite jade: greasy; amber: resinous]

 

 

 

 

[Pearl: pearly; tiger'seye: silky; granite: dull]

 

 

Transparency

 

Technically known as "diaphaneity", the degree of transparency of a gemstone is one of its most directly observable and familiar characteristics.

Transparency (or lack of it) is dependent on how much light gets through the gem, and is affected not only by the chemical and crystalline nature of the gem, but also by its thickness and, as in the case of luster, by inclusions, and its surface condition. In the discussion and examples that follow below, we will be looking at the "potential" maximum transparency of a species in general, rather than the actual transparency of any individual specimen.

When light hits the surface of a gem, there are only three fates for it (with respect to transparency). Various portions of the total amount of light will be reflected, absorbed or transmitted. The proportion in each category will determine the transparency of that gem.

 

 

 

[Three fates for light hitting a gem: it can reflect (be returned) from the surface or interior of the gem, it can be absorbed by the gem, or it can be transmitted through the gem]

 

Reflection: Light is reflected when it hits an exterior or interior surface of the gem and is bounced back off, or out of, the gem, in the direction of the observer.

Absorption: When light enters a gem and does not exit, we say it has been absorbed. Light is a form of energy, and energy does not just disappear, instead the visible light has been converted to a non-visible form of energy, in most cases, heat.

Transmission: Light that travels through the gem and exits in a direction other than that of the observer, is said to have been transmitted.

The issue of transparency (with the factors of reflection, absorption and transmission) is actually more complex than it may seem at first, because it is intimately linked with the color characteristics of a gem. For the time being, however; we can be satisfied with the following descriptions:

Opaque: No light is transmitted.

Translucent: Some light is transmitted.

Transparent: A high proportion of the light is transmitted.

The term "semi" is sometimes added to describe intermediates, and gives additional categories beyond the basic three.

 

 

 

[Citrine: transparent; Prehnite: semi-transparent; chrysoprase: translucent; sugilite: opaque]

 

Within any particular species of gem, it is often the most transparent pieces which are the most valuable. For example, in chrysoprase, shown above, which is generally semi- to fully translucent, one finds occasional pieces that are semi-transparent. These are greatly admired and sell for higher prices. The same can be said of nephrite and jadeite jades where price (within the same color) can escalate dramatically based on nuances of transparency. Likewise, in gems that are usually opaque, like the sugilite pictured above, the occasional semi-translucent to translucent piece (called "gel sugilite"), is highly prized.

 

Color

 

The color of a gem is determined by selective absorption of some of the wavelengths of light. We know that what appears to us as white (or colorless) light is actually made up of light of various colors. Issac Newton was the first to demonstrate this back in the 17th century.

 

 

 

[Image ]

 

Scientists in later years, were able to show that the color of light is a function of its wavelength. In the diagram above, a wave-form representation shows the relative distance from crest to crest (wavelength) of the various components of white light. Notice that these distances increase toward the red end of the spectrum and decrease toward the violet end. The wavelengths are very small, and we do not have everyday measurements to describe them. A nanometer (nm) is one billionth of a meter, and is an appropriately sized unit for this use. Using this terminology, then, the portion of the electromagnetic energy spectrum which our eye and brain interpret as light, extends from approximately 700 nm on the long (red) end to about 400 nm on the short (violet) end.

 

Visible Light Spectrum (nm)

 

  • 700 - 630 = red
  • 630 - 590 = orange
  • 590 - 550 = yellow
  • 550 - 490 = green
  • 490 - 440 = blue
  • 440 - 400 = violet

(For generations, students have been introduced to "Mr. Roy G. Biv", as a simple device for remembering the order of the colors in the light spectrum). Not to get too far afield from our subject matter, it is necessary to mention that this spectrum extends greatly on either side of the narrow visible range: into ultraviolet, xrays and gamma rays on the short end, and into infrared, microwaves and radiowaves on the long end. The little segment of it that we are concerned with in this class, not only powers vision, but also photosynthesis, and many other biologically relevant processes. It is also important to point out that the energy content of the various colors of light is related, in an inverse way, to their wavelength. That is, light of shorter wavelength is more energetic than light of longer wavelength.

Selective Absorption: The color of most objects, gems included, is a result of a process called "selective absorption". Let's take an example: suppose you have on a yellow shirt--> Why is it yellow? The fibers and dyes in it absorb only some of the wavelengths of the white light that hits them, primarily in the red, orange, green, blue and violet bands. The wavelengths that are left (the yellow ones) are reflected back to the eye of the observer whose brain interprets light energy of that wavelength, as what we call yellow. I'm sure you can see how a shirt could be greenish yellow or orangey yellow if wavelengths slightly shorter or longer than yellow were also reflected, and red or blue if it had a quite different pattern of selective absorption. With opaque objects it is the color of reflected light that we see, with transparent and translucent ones, the color we see consists of a mix of both their reflected and transmitted wavelengths.

Let's see if we can put together the information on transparency with that on color:

  • Transparency will depend on the relative proportion of light reflected, transmitted and absorbed by a gem. The color of the gem will depend on what is reflected or transmitted after selective absorption has removed some portion of the spectrum.
    • If none of the wavelengths are absorbed: the gem will be colorless if it is transparent, or white if opaque.
    • If equal amounts of each wavelength are absorbed: the gem will be grey.
    • If all wavelengths are absorbed equally and completely: the gem will be black.
    • In colored gems: we will see a mix of wavelengths which were not absorbed and which (depending on reflectance vs transmittance) will give us a colored tranparent, translucent or opaque gem.

Ok, so selective absorption determines color, but what, then, determines selective absorption, you ask? That is, why, precisely, do rubies look red and sapphires look blue? The basic answer is simple, and two-fold, and goes right back to the basic point previously made in Lesson 3 regarding all gem properties. Selective absorption in gems is determined by an interplay between their chemical makeup, and their three dimensional structure.

The atoms (or ions) which create color in a gem are called "chromophores". Some of the most common chromophores in gemstones are: atoms of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, nitrogen, and boron and their various ions.

(A new, undefined term, "ion", has crept in here, so let's deal with that). Atoms are made of smaller particles: protons, neutrons and electrons. The protons have a positive charge (+) and the electrons a negative one (-). In an atom, such as oxygen, or iron, or any other, the number of protons and electrons are equal making it, overall, a neutral body.

Suffice it to say, that chemical and physical events can, and do, occur that add or subtract electrons from atoms, making them into negatively or positively charged bodies called ions. Fe is the chemical symbol for a neutral iron atom, Fe+2 designates an iron ion (an atom of iron which has lost two of its electrons), Fe+3 has lost three, Cl- is a chlorine atom that has gained an electron, etc. The main point for you to understand is that events that occur in the "life" of gems and minerals (or in a gem enhancer's laboratory) can change the ionic state of their constituent atoms and ions, and thereby affect their color.

Back to the main point, the presence of various chromophores, as well as certain details of the three dimensional structure of the material itself, cause the selective absorption, which, in turn, causes color. To put it another way, both the presence of particular atoms and ions, as well as specific crytal "defects" such as missing atoms or extra ones, areas of compression or strain, can act as the agents of color in gems.

 

Idiochromatic vs Allochromatic Gems

 

With regard to the source of their color, gems fall into two categories: idiochromatic and allochromatic. Idiochromatic gems derive their color simply from the chemistry of their basic formula. Due to this fact, such gems will always occur in various shades of the same basic color. The other group (more common) are allochromatic, meaning that the chemistry of their basic formula does not cause any selective absorption so in the pure state, they are white or colorless. In gems of this sort it is tiny, trace amounts of impurities that act as the chromophores. Such gems occur in colorless forms as well as in a variety of other colors depending on the nature and amount of the "contaminants" in them.

 

[I think you'd probably get an argument from someone who is admiring their beautiful blue sapphire, if you called the tiny amounts of titanium and iron that give it that color,"contaminants", though.]

 

Some examples of idiochromatic gems are: peridot containing iron, (Fe), rhodochrosite with manganese (Mn) and cuprite and malachite containing copper (Cu).

 

Idiochromatic Gems

 

 

 

 

[Peridot (Fe+2), rhodocrosite (Mn), cuprite (Cu+1), malachite (Cu+2)]

 

Hey wait a minute, you say--> cuprite is red, malachite is green, and both contain copper! What gives? Welcome to the wonderful world of gem color! It is not quite as simple as: this element makes this color, and that element makes another color. Each gem's color is determined by an interplay between its chemical makeup (including the ionic state of its chromophores) and its structure.

To further pursue this point: some emeralds are green due to chromium content, while some get their green color from vanadium. So, iron (as in peridot), copper, chromium or vanadium can each be responsible for "greenness" in a gem. But on the other hand, chromium in corundum makes red rubies, and iron in chalcedony, makes orangey carnelian, but in sapphires gives us yellow. Futhermore, green zircons and green diamonds get their color not from chromophores, but from crystal defects.

 

Allochromatic Gems

 

Some examples of allochromatic gems are: beryl, corundum, quartz, grossular garnet, tourmaline, topaz, spinel and nephrite jade. In some cases the "pure" material is the most common and therefore the lowest in value (corundum, quartz, beryl and topaz are in this category); but in others, the pure form is so rare as to be a high value collector's item. This is especially true in the case of grossular garnet, tourmaline and nephrite jade. Colorless spinel is so rare that it literally has not been found in Nature; we know it can exist, though, because colorless synthetic spinel is made in labs.

A good example of an allochromatic gem species is corundum. Pure Al2O3 is colorless, as in white sapphire, but if we add just a tiny bit of iron to the mix then we get yellow or orange fancy sapphire, pair the iron with a bit of titanium, and the gem is the familiar blue, and if chromium is the chromophore, then the corundum is red and called ruby.

 

Allochromatic Gems (in their pure state)

 

 

 

 

[Colorless beryl (Goshenite); "white" sapphire, colorless quartz (rock crystal); colorless grossular garnet (leucogarnet)]

 

 

Allochromatic Gems (in their impure state)

 

 

 

 

[Beryl: emerald (chromium or vanadium); corundum: sapphire (titanium and iron); quartz: carnelian (iron); garnet: Spessartite (manganese)]

 

Other Sources of Color: Some gems get their color (or apparent color) from visible to microscopic inclusions of other minerals within them. One of the most beautiful of all the chalcedonies, often called "gem silica" but more properly termed "chrysocolla chalcedony" has a vivid blue-green color. The minute quartz crystals are actually colorless, but in amongst them are tiny crystals of the blue green (very soft) mineral, chrysocolla. The overall impression, in the best specimens, is a translucent chrysocolla colored gem, with the durabililty of quartz.

In the varieties known variously as strawberry and raspberry quartzes, visible particles of red or red-orange hematite in the colorless quartz, create a pink, orangey, red or polkadot looking gem, depending on their size and number.

 

Inclusions

 

 

 

 

[Chrysocolla quartz, strawberry quartz]

 

 

Patterns in color: banding/zoning

 

One of the most common features of some of the aggregate gems is the presence of patterning. Since these gems are formed from very tiny single crystals, we can easily envision conditions where differently colored pools or batches of tiny crystals mix and intermesh creating bands, dots or other patterns. Agates and jaspers are the most commonly seen gems with strong patterns.

It frequently happens that single crystal gems subjected to changing conditions during their growth can also show bands or zones of different colors or shades of the same color. When these are dramatic and attractive, they are desirable, but far more commonly, gems of this type have nondescript, patchy, or zoned coloration, and are considered inferior to more evenly colored pieces.

 

Aggregates With Patterns

 

 

 

 

 

 

[Zebra agate, picture jasper, Mookaite jasper, carnelian, lavendar agate, Dalmation jasper, rain forest jasper]

 

 

 

 

 

Single Crystal Gems with Attractive Color Zoning

 

 

 

 

[Ametrine, multi-color tourmaline, watermelon tourmaline]

 

 

Color Descriptions in Colored Gemstones

 

There are three aspects to a formal colored stone color description: hue, tone, and saturation. Using these three descriptors, very detailed and nuanced color discriminations can be made, and communicated, between gemologists, jewelers and gem buyers. Let's take them each in turn:

Hue: The hue of a gem is its basic position in the color spectrum: red, orange, yellow, green, blue or violet--> but it also includes all the possible intermediates like slightly yellowish orange, or moderately bluish green.

Tone: The tone of a gem, basically how light or dark the color, is independent of its hue and ranges from so light as to appear virtually colorless, to so dark as to look black.

Saturation: The least commonly quantified aspect of gem color is "saturation", which is a measure of the purity of color, that is, the relative presence or absense of modifying grey or brown hues. It turns out that in most cases, as long as the hue and tone are reasonably nice, it is the degree of saturation of color that is the prime value setter in gemstones.

You might ask, why does color description need to be so formalized? The main reasons are listed below:

  • Small color differences mean big dollars!: in the rarified world of gem and jewelry connoisseurs, zeros can be added to prices based on what look like small differences in color to the rest of us.
  • Commonly used adjectives are subjective, and culturally based: Without some system of regularizing color descriptions it is very difficult to communicate color information efficiently.
    • For example: Give three of your friends a unlabeled color chart and ask them to show you "royal blue" or "lime green", or medium orange. I can guarantee you'll get three noticeably different color choices from each of them. Additionally, limes may not be a familiar fruit in the country where you wish to purchase a gem, or royalty there may be associated with yellow, not blue.
  • Color memory is notoriously unreliable. Without a system whereby precise color coordinates can be recorded, there is little chance of doing a good job matching a new piece to an existing one.
    • A couple of simple exercises can verify this statement: You have a nice sunny yellow paint in the kitchen, but some of it is chipped off and needs repair. You go to the paint store and pick the color from the paint charts that matches the color in your memory--> what are the chances it will, in fact, match? Or, let's say you want to buy a new shirt to match your favorite brown pants: good luck, if you don't wear or take those pants with you when you go shirt shopping!

     

    GIA Color Description/Grading System

     

One well done, and widely used, system for color description is that developed and taught by GIA (Gemological Institute of America). Although not universal, it is familiar world-wide, and the basis for most formal gem description and evaluation in the US and Europe.

Since the wavelengths and light colors grade into one another in infinitesimal changes, there are an essentially infinite number of hues which could potentially be described. Most of these hues would be indistinguishable from each other to our eyes, so GIA has settled on a group of 31 which humans with normal color vision (and some training) can discriminate. The set of plastic gem models below is a representation of those 31. (It should be noted that GIA has taken some liberties with the traditional "Roy G. Biv" spectral colors, deleting indigo, and adding purple after violet. In the set below, then, you see: red, orange, yellow, green, blue, violet and purple. The intermediates are described by terms like slightly, moderately and strongly to indicate a spectral hue modified to various degrees by those on either side of it on the spectrum. It also recognizes hues which are exactly 50/50 mixes such as red-orange and blue-green.

 

HUE

 

 

 

 

[Gia's 31 basic gem hues]

 

The hue, then, consists of the key (or spectral) color plus adjectives describing the hue and strength of the secondary modifying color, if any. For example: slightly purplish red, symbolized by "sl p R" (sl for degree, lowercase p for the secondary hue (purple), and upper case R for the primary hue, (red). The description strongly yellowish green: "st y G" would be decoded using the same logic. Once you have been trained to see nuances of color, you recognize that pure spectral colors in gems are quite rare, and as a result, costly. For example: a hue of simply "B" would be pure spectral blue and if other factors of color and clarity were good, the piece would command a premium price.

If you are thinking that it looks like there are more gradations in the blues and greens in the display above, than in the other colors, you are right. Human vision has finer powers of color discrimination in that part of the spectrum, which this system takes into account.

 

TONE

 

Each of the 31 hues exists in a range of tones from almost colorless to almost black. GIA labels the tones as 0 - 10. {0 ( appears colorless), 1 (extremely light,) 2 (very light), 3 (light), 4 (medium light), 5 (medium), 6 (medium dark), 7 (dark), 8 (very dark), 9 (extremely dark), 10 (appears black).

The figure below represents the 2-8 part of that range, which is, in the great majority of cases, the range for marketable colored gems. For most species the most valuable tones are in the 5-6 range. The set below is shown without hue, and it takes practice and patience for the would-be colored gem grader to learn to superimpose hue onto these, and get a valid tone reading.

An additional complication comes from the fact that gem species differ in their inherent tone ranges. For example, let's compare an aquamarine and a pyrope garnet each of tone 6. Objectively, each is exactly the same, but that depth of color is about the deepest that will ever be found for aquamarine and the about lightest possible for any pyrope. One should not be surprised, then, to find the aqua dealer calling her stone "very dark" and the garnet seller raving about how beautifully light his stone is when they are both "6"'s.

 

 

 

[GIA's tone scale from 2 (very light) to 8 (very dark)]

 

 

SATURATION

 

Finally, it's time to examine the most subtle aspect of gem color, saturation: in a manner of speaking, this measure is the degree to which the other spectral colors "muddy up" the main hue. Think of a can of pure red paint and start adding in various amounts of all the other colors--> the more you add of the other spectral hues, the "browner" the red will get. Now do the same thing with a can of pure blue: the more you add the "greyer" the blue will get. In general, desaturating "warm colors" makes them look brownish while the same effect in "cool" colors looks more grey. Therefore, GIA's system of describing saturation makes a distinction between cool and warm hues.

 

Warm hues = green through red (desaturated to brown) Cool hues = purple through blue (desaturated to grey)

 

Six degrees are recognized ranging from: 1 (brownish/greyish), 2 (slightly brownish/greyish), 3 (very slightly brownish/greyish), 4 (moderately strong), 5 (strong), 6 (vivid)

**In the figures below, you can get a better idea of the saturation effect by looking at the flat end of the plastic gem replica, rather than the "gem part".

 

 

 

[GIA's six degrees of warm hue saturation]

 

 

 

 

[GIA's six degrees of cool hue saturation]

 

When giving a gem's formal color description in words, then, the gem below might be said to be: medium dark, slightly greyish, blue-violet. It sounds more natural to put the tone, saturation and hue in that order. In a numeric description as required in offical gem grading documents, however: the order would be: hue, tone and saturation, thus: BV 6/2

 

 

 

[Iolite: in words: medium dark, slightly greyish, blue-violet = official "grade": BV 6/2]

 

One final point on the GIA color grading scheme: Two non-spectral colors are used (in addition to the officially sanctioned 31) and those are pink (pk) and brown (br). If one were to strictly follow the GIA system, all shades of pink are really lighter tones of red, and brown is simply desaturated orange. It is rather a matter of bowing to tradition and convenience to recognize pink and brown as "colors" in their own right. You will see evidence of this practice in the color description below.

 

 

 

[Spinel: medium dark, moderately orangey, strong pink--> mod o PK 6/5]

 

 

Color Grading in Diamonds

 

You will recall from Lesson 1 that within the gem industry, there are separate systems for marketing, grading, and describing colored gemstones and diamonds. For virtually all natural diamonds, discernable color is a negative attribute. The closer it is to an absolutely colorless condition, the more highly valued is the gem.

In the case of what are called "fancy" diamonds, whose color is both intense enough, and attractive enough, to be desirable, color is described and evaluated in a similar manner to that used for colored stones. There is sort of a "U" shaped value curve for diamonds, whereby the highest values accrue to only the whitest, and then, again, to the most vividly colored specimens, with value bottoming out in the central ranges where there is just a bit, to a moderate amount, of color.

Although most of the diamonds you might see on a day-to-day basis are called "white" and appear so, a little study and comparison will verify that a truly colorless diamond is a thing of great rarity, and the vast majority of diamond gems are actually tinted with small but noticeable amounts of yellow or brown.

I was taking some liberties, perhaps, by using the GIA system in the preceding discussion of colored gem descriptions, but without doubt, the GIA system is the one to learn if you are interested in diamond colors and value. It is understood everywhere in the world, and used for formal grading in most countries. Even competing gem grading laboratories either use the GIA system, or provide a key to translate theirs to it. For example AGS, The American Gem Society uses 0 - 10 for their color grading scale (0 = D, 0.5 = E, 9.5 = W, 10 = X - Z, etc.), but gives the customer an exact conversion scale to the GIA system with their reports.

Before the GIA system was developed (beginning in the 1930's), there were as many diamond color descriptions as there were diamond sellers. Many of them used A, B, C and A+, AA, AAA etc. while others used adjectives like "river" and "cape". It is easy to see how difficult it would be to have a reliable system for trade under those conditions. GIA's scale did away with A, B and C because of their long histories and diverse useages, and developed a system based on color grades from D-Z for "colorless" stones, plus the term "fancy" to indicate those whose strong color made them more, rather than less, valuable.

 

What the Letters Mean

 

D, E, F: gems in this range appear colorless even in larger sizes, only a highly trained diamond grader can tell the differences between them.

G, H, I: these grades describe gems that look colorless to most viewers in smaller sizes and if mounted.

J, K, L: small and mounted stones of these grades look near colorless, but larger and unset gems begin to have noticeable color

M-Z: gems in this range are worth much less than higher color grades and range from some color noticeable to distinctly light yellow (or brown).

Z+: beyond Z is the range of the "fancy" diamonds whose value is based on their hue tone and saturation, as in colored stones. In general browns are least valuable with yellow, orange, and green worth considerably more. The pinnacle of value for naturally colored diamonds is occupied by purple, blue, pink, and at the very tip-top, red.

The images below may or may not be enlightening, based on the characteristics of your vision, viewing circumstances, and monitor calibration, but they will hopefully serve to illustrate at least some of the aspects of our topic.

 

 

 

[Image courtesy of www.indiangemstonelab.com]

 

 

 

 

[Image courtesy of S.S. Gems, Jewelers]
Amritsar

 

 

How Do they Do That?

 

After straining your eyes to see the minute changes in the illustrations above, you might ask, how can a grader do it, especially when a great deal of money rides on the difference between, say an F and a G grade, or an L and an M? I might joke that the answer is "verrrry, carefully". In actuality, not everyone can become a successful diamond grader--> there is both exacting training and "raw talent" involved.