Optical properties of gems : 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 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.
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]
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.
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.
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).
[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.
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.