Mineral Optical Properties

The Properties of Light

Electromagnetic Energy

Light is part of the electromagnetic spectrum, one of the fundamental energies of the universe. It's present everywhere in substantial amounts.

Light energy travels in waves. The energies on the electromagnetic spectrum differ simply by their wavelengths. Some are shorter, some longer. Wavelength has nothing to do with the amount of energy carried. Wave amplitude determines that. All wavelengths can be high or low in energy.

Although, in nature, all electromagnetic energy is essentially the same, varying wavelengths have different characters. For example, we relate to some as radio waves, others as X-rays. We see a very tiny part of the electromagnetic spectrum as light. Within this section, we perceive different wavelengths as colors. When we see all the wavelengths at once, we describe it as white light. The color black is a lack of electromagnetic energy in this portion of the spectrum.

Just beyond the section of the visible electromagnetic spectrum, we find ultraviolet and infrared light. Some animals can see these light frequencies, but humans can't. We can sense them, but not with our eyes. Ultraviolet can give us sunburn, and we feel infrared as heat.

Simply put, light is a form of energy. One color varies from another by the length of the waves of energy coming at us. Please note, colors are how our brains interpret these wavelengths. Color has no relevance outside human experience.

Refraction

Light travels at 186,282 miles per second in the vacuum of space. That's really fast. Although the speed of light doesn't change perceptibly in the Earth's atmosphere, it does slow when it enters a dense medium like a crystal. When light passes from one medium to another, it slows down and changes direction or bends. The property we call refractive index (RI) measures how much it changes.

In mineralogy, refractive index is the relationship of the speed of light in a gem to the speed of light in a vacuum. If a mineral has an RI of 1.655, that means that light travels 1.655 times faster through space than it does through the mineral.

Fortunately, mineralogists don't need to measure the speed of light. Instead, they measure RI with a refractometer. The mineral is placed on a special surface, and light is passed through it. This casts a shadow on a scale. Where the shadow line falls on the scale marks the RI. Much simpler than attempting to measure the speed of light, right?

Knowing a mineral's RI is one of the most important pieces of information you can get when identifying a specimen. It relates to many of the topics discussed below.

Polarization

Usually, light disperses evenly in every direction. For example, if you light a candle in the middle of a room, the brightest area is right next to the candle. The further away from the candle, the less light there is. If you could follow a single ray of light, you'd see that it spreads equally in every direction as it travels.

When light passes through certain substances, it becomes polarized. That means it has sideways vibrations at right angles to each other. As normal light, it emanates in every direction. When polarized, light only moves in linear directions (at right angles).

Crystals frequently polarize light, which accounts for some of the optical phenomena we see. It's an important clue for mineral identification.

Selective Absorption

When you see all the wavelengths of visible light at once, you perceive it as the color white. If you can only see a portion of the visible spectrum, you perceive a hue. Simple enough. However, the physics of light is a bit more complex.

If you were to remove just a little bit of the white light spectrum, there would be amounts of several different hues left. When this happens, our eyes average the wavelengths together. We perceive these partial spectrums as a single hue, even though there are actually several wavelengths of light coming at us at once.

When light enters a crystal, it absorbs a portion of the light. Thus, the light exiting the crystal is no longer white. Instead, we see a hue. This process is called selective absorption.

How Does Color from Selective Absorption Differ From Pigment?

We add pigments to materials to produce or change colors, like dye to your clothes or food coloring to icing. Selective absorption doesn't add anything to a crystal. Most crystals themselves are colorless. Only the light passing through them, the color we see, changes.

For example, a streak test on most minerals, such as quartz or calcite, will leave a colorless streak.  Why? There are a few reasons. Many minerals are composed of light-colored or transparent crystals, and when powdered, the individual particles scatter light in a way that results in a white or colorless appearance. The reason is that the fine particles of the powdered mineral interact with light differently than the solid crystal, often appearing white due to diffuse reflection.

Furthermore, as mentioned above, the color in many minerals is due to impurities, surface coatings, or inclusions. When ground into a powder, these impurities are dispersed, and the true "base" color of the mineral dominates, often white or colorless.

The Spectroscope

We can see the spectrum of light with an instrument called a spectroscope. It separates white light by wavelength. If you look at sunlight through a spectroscope, you'd see all the colors of the rainbow nicely spread out. However, if you look at the light coming out of a colored gemstone, you'd see dark lines here and there. These are the areas of selective absorption.

Selective absorption patterns are sometimes diagnostic. Thus, you can use them at times to distinguish one gem from another.

To the right, you can see the absorption spectra of white light, ruby, and garnet as viewed through a spectroscope. (Note that white light has no dark lines). When held in your hand, rubies and garnets look very similar. Nevertheless, their spectrums vary considerably. While the garnet spectrum has just a few lines, the ruby spectrum has large areas of absorption as well as several distinct lines.

Although the spectroscope is an important tool for mineral identification, it does have limitations. Several minerals share patterns. Take minerals colored by the element chromium. The presence of chromium creates the same or very similar absorption pattern, whether it's in a ruby, tourmaline, diopside, or other mineral. On some occasions, a spectroscope can make a critical distinction. Generally speaking, however, you'll get more useful information from other instruments.

Color Change

While sunlight contains all the visible wavelengths of electromagnetic energy, other forms of light don't. They have peaks and valleys, rather than an even distribution of electromagnetic energy. For example, our electric lights appear white, but that's a result of our eyes averaging the wavelengths together.

The hues we usually see in gems depend on having a full spectrum of white light present. If you have less than that entering the gem, what comes out may be different. This is why some minerals, like alexandrite or certain garnets, change color between natural and artificial light.

Refractive Index

Have you ever noticed how ice cubes in a glass of water nearly disappear? If they have rough surfaces, you can see them, at first. Just like unpolished gems, the rough ice surfaces scatter light in numerous directions. As the edges of the ice melt and become smooth, the ice cubes become invisible. Just a hint of their outlines, air bubble inclusions, and cracks remain to tell you the ice is there.

(IMPORTANT NOTE: Now, we're getting into the heart of mineralogy. The following discussion has applications to mineralogy, even if the examples focus on gem cutting.)

Water and ice have the same RI. While we know light will bend when it passes from one medium to another, when both substances have the same RI, speed and direction don't change. (That's the only exception to the rule). No alteration of the speed or direction of light takes place between the water and the ice. You can clearly see the air bubbles, though, since they have a different refractive index. The cracks also disturb or reflect light. They act as mirrors, scattering light. Hence, they show up plainly as they interfere with the free passage of light.

We know the technical definition of refractive index: the relationship of the speed of light in a gem to the speed of light in a vacuum. However, the practical implications of this have much greater significance for mineralogy. You've surely seen how a straw in a glass of water appears to bend at the surface. The difference in RI between the air and the water causes this. Understanding how this bending works and how much light bends when passing from air to gem is essential when cutting gems for brilliance.

Inclusions and Refraction

Inside gems and minerals, inclusions may act like straws and air bubbles in water. If the inclusions are transparent, how much they stand out relates directly to how much their RIs vary from the surrounding material. For example, if an amethyst contains small crystals of clear quartz, those inclusions would be "low relief," nearly invisible. Clear quartz has the same RI as amethyst. However, if the amethyst contains inclusions of a mineral with a very different RI, they would show up clearly, in "high relief."

Fractures and Refraction

Fractures in a transparent mineral also disrupt the flow of light. Gem cutters must carefully place facets to apply discipline to the internal flow of light. After all, this may increase the "eye appeal" of the gem. Fractures can disrupt this discipline and lead to loss of brilliance and scintillation and, thus, reduce the price and desirability of the gem.

Filling a fracture with a substance of the same RI as the gem can minimize its light-disruptive effect. Just as light can pass through water, ice, and water again without bending, so can it freely pass though several layers in a gem. Of course, provided they all have the same RI.

Oiling and Refraction

Gem cutters most commonly use oil to fill fractures in gems. Mineral and vegetable oils have RIs close to the lower RI range of gems. Epoxy resins are also used. Filling gem fractures is an ancient and very low-tech procedure. Simply place the stones in oil and keep them warm until they absorb the oil. Since emeralds are almost always fractured, they commonly undergo this procedure.

Fracture filling can do wonders for a gem's appearance. However, these gems require special care. Subjecting these stones to hot and soapy water can remove the fillings and all their optical benefits.

Immersion Tests and Refraction

The more involved you become in mineralogy, the more you'll use optical properties like refraction in multiple ways. For example, mineralogists may immerse minerals in fluids with RIs as close to the mineral as possible. This allows a much clearer and easier view of the specimen's interior. Also, you can estimate the stone's RI. While not precise, this method can clearly reveal if the stone is close or far from the liquid's RI.

Estimating Refractive Indices of Immersed Gems and Minerals

The following pics show three gems/minerals: an amethyst, RI 1.54; a garnet, RI 1.76; and a cubic zirconia (CZ), RI 2.15. See what happens as they are immersed in liquids with progressively higher RIs.

Viewing Inclusions in Immersed Gems and Minerals

In the photos below, you'll see another example of how immersion can help you identify gems and minerals. Just from looking at the picture, you can see that both these stones have about the same RI. Although you can't tell for certain if they're the same species, you know they're not radically different, like quartz and sapphire.

When immersed, the stones reveal inclusions previously invisible. The stone on the right is heavily included, while the other is relatively clean. In conjunction with microscopic examination, this technique can help show otherwise invisible inclusions and dye treatments. (See "How to Test for Diffusion Treated Gems" for more information).

For a simple and fun experiment, try this. First, take a clear vegetable oil from your kitchen. Next, find a gem with a low RI. Quartz (including amethyst and citrine), emerald, aquamarine, and iolite make good choices.

Put enough oil in a small vial or drinking glass to cover the gem. Don't immerse it yet. Look at the gem carefully with a good source of illumination. Then, immerse the gem in the oil and examine it again. What you can see now will astonish you!

Critical Angle

If you've ever spent a day at a lake, you've probably noticed a refraction effect known as critical angle. During the day, sunlight passes into the lake. However, for a moment just before sunset, the light reflects off the water instead of passing into it. Looking in the direction of the sun becomes difficult because of the glare. Whether sunlight passes into the water or reflects off it depends on the angle at which it hits the water. When the light reaches the critical angle, the surface reflects it.

The critical angle effect occurs with minerals, too, and, with gems, affects gem faceting in particular. If you haven't already experienced this, please try this little experiment. While facing the sun, look through a quartz crystal. Turn it this way and that to see as much of the interior as you can. Most of the time, you're looking all the way through the crystal. Once in a while, though, one of the sides will act as a mirror and reflect the sun towards your eyes. That happens when the light reaches the critical angle in your field of view.

Brilliance

Gem cutters must understand critical angles to bring out maximum brilliance in their cuts. A gem's brilliance, in essence, is the light that is reflected off the bottom or pavilion facets back towards the viewer's eye. From the above examples, you should understand that the relationship of the light to the bottom facets is below the gem's critical angle.

In theory, the higher the RI, the greater the potential brilliance of the gemstone. However, in actuality, brilliance involves so many other factors that this rarely has any practical value. For example, a gem not cut to ideal proportions or not polished well will have significantly less brilliance. Yet another factor is color saturation. The deeper the gem color, the more light is absorbed. Thus, the brilliance suffers.

Try this experiment. Get yourself some well-cut and polished quartz and aquamarine gems. (Cut plays a significant role in a gem's brilliance). Quartz and aquamarine have very low RI's. On the other hand, diamond has one of the highest. Carry your well-cut quartz or aquamarine around with you. You may find they have more brilliance than some of the diamonds you'll see!

Scintillation

Gemstone scintillation, a multitude of tiny flashes of light, is a property closely related to brilliance. The number and orientation of the facets determine the amount of scintillation. The gem's crown facets break up the light of the pavilion facets, causing the flashes. While gems with high scintillation usually have high brilliance, this isn't always the case.

Windowing

Recall looking through a quartz crystal to find the critical angle? You can also see another optical phenomenon during that test. Windowing is when you can look straight through the crystal, without light reflecting off a side.

Economics has more to do with gem cutting than aesthetics. That's just a fact of life. Lapidaries cut most gems to achieve maximum size when finished, rather than maximum beauty.

Now, that often requires cutting the pavilion facets at angles well off the critical angle. When this happens, you'll see windowing in the finished gem.

With the gem face up, you'll see the deepest color and scintillation around the edges. However, you can look right through the center, the windowing area. Here, no light is reflected back, hence no sparkle, and the color is much lighter.

If you're unfamiliar with windowing, take the time to look for it. Once you see it, this effect becomes quite obvious.

Small windows are common and have little effect on a gem's beauty. Large windows, however, mar the appearance of a gem significantly. Customers will take that into consideration when looking to make a purchase.

Double Refraction and Birefringence

When light enters a mineral, it's refracted. If the light faces no restrictions other than slowing down, scientists call the gem isotropic. In isotropic materials, light passes every direction at the same speed and with the same color. This occurs in amorphous materials like glass, plastic, opal, and amber, as well as minerals that form in the cubic system. These materials have a single RI and a single color.

In the other five crystal systems, light becomes polarized. Thus, it vibrates in two or three planes. Each direction of light has a different speed and RI. This is called double refraction. Birefringence is the difference between the refractive indices of a gem. For example, if a stone has a high RI of 1.623 and a low RI of 1.617, the birefringence is 0.006.

While some gems have three angles of refraction, measuring all three RIs in a faceted gem is difficult. So, simply measuring the range between the high and low RI suffices, rather than distinguishing all three. Thus, gemologists call all gems, other than those in the cubic system or amorphous, doubly refractive. Each angle of refraction can have a different color. (Of course, colorless gems are the exception, since they don't have multiple hues). Few gems show three colors, so this is where the third refraction angle becomes important. (I'll cover this in more detail under "Pleochroism").

Scientists call stones with double refraction anisotropic. That means an unequal distribution of gemstone optical properties in a crystal. The term anisotropic also applies to physical properties, like minerals that vary in hardness.

Doubling

In most doubly refractive gems, the difference between RIs is so small you can't see it. You can still measure the difference with a refractometer. However, some gems have such a great difference between RIs it causes doubling or multiple images. For example, calcite has a birefringence of 0.172 and shows extreme doubling.

High birefringence is common in a few other gems, notably tourmaline and zircon. When looking into them, you'll see a doubling of the back facets. This tells you immediately that you have a gem with high birefringence. Since this isn't an optical property many gems possess, it's an important clue for gem identification. However, not seeing strong doubling doesn't mean a gem has low birefringence. If you're looking straight down a gem's C axis, you may see only single refraction. Sometimes, you'll need to look through a side to see the doubling.

Crystal Habits and Refraction

A quick review of crystal habits will help explain why some crystals are singly, doubly, or triply refractive.

Amorphous

Amorphous materials have no crystal structure or habit. They have a single refractive index and a single color.

Isometric

Because of the equality of the axes, minerals in the isometric or cubic system are singly refractive.

Tetragonal

Tetragonal minerals, and the gems cut from them, will have double refraction. Light passing through the C axis will be refracted at a different speed than that on the shorter axes. If colored, tetragonal gems will also vary in hue. Obviously, colorless crystals won't show two hues.

Hexagonal

In the hexagonal system, light will refract at two speeds, one on the optic axis and another on the shorter, but equal length axes. If the gem is colored, the two directions will show at least slightly different hues.

Orthorhombic

In the orthorhombic system, although all the axes meet at 90°, they all have different lengths. Hence, all three optic axes will refract light at a different speed. Orthorhombic gems can potentially show three different colors.

Monoclinic

In the monoclinic system, all three axes are different lengths but only two meet at 90°. Thus, each optic axis will refract light at a different speed. Each can potentially have a different hue.

Triclinic

In the triclinic system, none of the axes have the same length nor meet at 90°. Again, each optic axis will refract light at a different speed and each can display a different hue.

Summary

Only amorphous materials or minerals in the isometric system have a single refractive index. They have a uniform color. Minerals in the tetragonal and the hexagonal system have double refraction. They can also show two different colors. The other crystal systems, orthorhombic, monoclinic, and triclinic, have three unequal optic axes. Each will have a different angle of refraction and potentially three different colors.

Pleochroism

Pleochroism means showing multiple colors and results from birefringence. You'll also encounter the term dichroism, which means two colors, or trichroism, which means three colors. Just as RI varies by the optic axis, so does the color in many occasions. Often, the color difference is so slight you'll need a dichroscope to see it. For example, see the image to the right.

With other gems, the difference is so extreme you can see both colors at the same time. Andalusite, with its green and orange pleochroism, is an excellent example. You'll also find extreme pleochroism in tourmaline and iolite. If properly cut, iolite displays blue to violet face up, but pale yellow or colorless from the sides. (Be aware, iolite's other properties come close to amethyst. The two are easily confused. However, a quick look at the gem from the side will distinguish it).

Don't confuse pleochroism with color zoning. Some stones, like the ametrine pictured to the right, may have separate areas of color on the same optic axis. This is entirely different from pleochroism.

Dispersion

Dispersion is the spreading of white light into its component colors. When you see a rainbow coming out of a prism, you're looking at dispersion. The fire of a diamond is another example. Although common to most gems, dispersion is rarely strong enough to be easily seen.

Dispersion is easy to confuse with birefringence. With dispersion, each wavelength of light is refracted slightly differently. However, this has nothing to do with the gem's optic axis. Diamonds, which crystallize in the cubic system and are singly refractive, have high dispersion, like that show in the photo to the right.

Dispersion can be an important piece of information for gem identification. You'll rarely see dispersion under 0.020, and that would be weak. So, if you see noticeable dispersion in a gem, you'd eliminate quartz (0.009), sapphire (0.018), and most other gems as candidates.

The amount of fire, or dispersion, you see from a gem depends to a great extent on cut. The pavilion angles determine how much light is reflected back to the top. On the crown, high angles emphasize dispersion, whereas low angles or a thin crown will show considerably less. This explains why some diamonds appear to have much more fire than others. All diamonds have the same gemstone optical properties, including the same dispersion. However, they don't display it equally.

When you become familiar with how much dispersion a well-cut diamond will display, you can use that to distinguish it from CZs. Diamonds have a dispersion of 0.044, very high in the world of gems. However, CZs have much higher dispersion at 0.060. When you know what you're looking for, you can see it from a distance.

Luster

Luster is how much light is reflected off a smooth surface. This includes light from the pavilion facets as well as the surface. Diamonds have the highest luster of all gemstones: adamantine. In fact, adamantine literally means "diamond-like." Gems such as rubies and sapphires have sub-adamantine luster. Most transparent gems you'll encounter will have vitreous or "glass-like" luster.

Since photos don't always capture luster well, you need to examine some well-cut diamonds and colored stones to see the differences. Nevertheless, the picture below will give you some idea of the difference between a diamond with adamantine luster (center) and vitreous colored stones (left and right).

Other types of luster include dull, metallic, pearly, silky, waxy, resinous, and greasy. Although the terms are somewhat subjective, they are pretty much self-explanatory. Amber is usually resinous. Hematite is metallic. Charoite and coral are waxy. Also, gems can display more than one type of luster. For example, some stones can be either sub-adamantine or vitreous. Chalcedony is usually vitreous but can also be greasy. Opal is usually vitreous but can be resinous. Some gems can display different lusters based on the orientation of its optical axis.

Remember, consider the quality of lapidary work when assessing luster. Just because a stone shows a dull luster doesn't necessarily mean that's an actual property of the gem itself. It could just lack a decent polish. Conversely, stones like amber can be taken to a vitreous level of polish.

Optic Character

Although polarization splits light in two directions, not all the individual beams are polarized. Gems in the hexagonal and tetragonal systems only polarize light in one direction. The light passing down the optic axis remains unpolarized. This is the definition of a uniaxial gem. Sapphire, quartz, tourmaline, and zircon are common examples of uniaxial gems.

Other doubly refractive gems have two optic axis, both of which are polarized. They are called biaxial gems. Since this occurs with all minerals in the orthorhombic, monoclinic, and triclinic systems, they're more common. You can see a biaxial optic sign in the photo to the right.

Scientists also distinguish optic signs as positive, negative, or without sign. For an explanation of these terms, see part 3 of our refractometer guide.

Fluorescence

The molecular structure of a crystal changes the energy that comes into it. We've seen that with selective absorption. This also happens with the invisible portion of the electromagnetic spectrum. In particular, if you put ultraviolet energy (UV) into a gem, some of it may change into visible light. This is called fluorescence. The doublet pictured to the right displays this. (Its top is inert).

Some gems will continue to emit visible light after the UV is turned off. This is called phosphorescence.

When testing a gem for fluorescence, gemologists use two different frequencies, longwave (315 to 400 nm) and shortwave (200 to 280 nm). You'll often get different results from the different frequencies, so test both ranges. During testing, note the hue (if any), intensity (weak, moderate, strong, or inert), and if the color is evenly distributed, zoned, or patchy.

Fluorescence and Gem Identification

If you've already tested a gem's RI and optic sign, an ultraviolet test may help you narrow the possibilities. Don't start your gem identification process with testing fluorescence, since most mineral species have specimens that are inert to ultraviolet. However, if one of your candidates shows green or orange, weak or strong, this is useful information. Ultraviolet testing is easy to conduct, though it poses some health risks, too. See "Ultraviolet Testing and Gemstone Identification" for more information.

Fluorescence and Gem Grading

Fluorescence can play an important role in gem grading. For example, diamonds fluoresce a variety of hues and at different strengths. Since sunlight contains a lot of UV, it will often affect their appearance. However, since 2008, the standard for diamond color grading is daylight-equivalent lighting with a UV component, so diamond color grades assigned since then should already account for the fluorescence factor.

Burma rubies are also known for intense red fluorescence. This gem literally glows in sunlight.

In addition, oils used as fillers in emeralds and other gems often fluoresce. Checking for fluorescence is a great way to detect this treatment, since these fillers are nearly invisible in normal light.

Phenomenal Effects

Light does some wondrous things inside special gems. It can create stars and cat's eyes, billowing clouds, and intense multicolored reflections. These special optical effects are why we cherish moonstones, pearls, opals, sunstones, alexandrites, labradorites, and other phenomenal gems. See "Phenomenal Gems" for more information.

Optical Properties Self Test

See if you can define these terms related to optical properties. This article contains all the answers.

1. Anisotropic

2. Birefringence

3. Brilliance

4. Color Change

5. Critical Angle

6. Dichroscope

7. Dichroism

8. Dispersion

9. Double Refraction

10. Electromagnetic Energy

11. Fluorescence

12. Isotropic

13. Optic Character

14. Phosphorescence

15. Pleochroism

16. Polarization

17. Refraction

18. Refractive Index (RI)

19. Refractometer

20. Scintillation

21. Selective Absorption

22. Spectroscope

23. Trichroism

24. Ultraviolet

25. Windowing

Donald Clark, CSM IMG

The late Donald Clark, CSM founded the International Gem Society in 1998. Donald started in the gem and jewelry industry in 1976. He received his formal gemology training from the Gemological Institute of America (GIA) and the American Society of Gemcutters (ASG). The letters “CSM” after his name stood for Certified Supreme Master Gemcutter, a designation of Wykoff’s ASG which has often been referred to as the doctorate of gem cutting. The American Society of Gemcutters only had 54 people reach this level. Along with dozens of articles for leading trade magazines, Donald authored the book “Modern Faceting, the Easy Way.”

View All Articles

International Gem Society

View All Articles