Opal is a well-known example of a mineral whose color is caused by a physical phenomenon called “diffraction.” Other phenomena include iridescence, a rainbow effect seen in iris quartz and pearls; chatoyancy, which we see in cat’s-eye stones and some malachite; asterism, which is displayed in star stones; aventurescence, as seen in aventurine quartz and sunstones; adularescence, seen in moonstone; and play of color, or the alexandrite effect, seen in the alexandrite variety of chrysoberyl and some garnets. In every one of these groups, the cause of the color is related to some internal physical structure and not a metallic impurity or element in the mineral’s structure.

Opal Color

For centuries, people tried to explain the play of color seen in many opals. Finally, in the 1960s, we developed equipment that could actually see the internal structure of opal. It revealed a very orderly arrangement of submicroscopic spherules of silica. These spherules and the spaces between them acted as a diffraction grating, spreading light into its various colors. The sizes of these spherules and the angle the light struck them, coupled with the viewer’s angle, determined which color wavelengths were canceled and which ones were reflected. Diffraction of light results in opal’s play of color.

Labradorite Color

A more common mineral that gets its play of color from diffraction is the feldspar mineral labradorite. This mineral can develop in huge formations, resulting in outcrops that give off flashes of color.

Labradorite crystallizes in thin wafers in parallel layers that repeat to form a diffraction grating. This has the effect of separating light into its colors, giving labradorite a play of color that depends, in part, on the angle of the source of light. The thickness of each crystal and each cluster of crystals in their parallel layers also affect which color is seen. Labradorite can flash bronze, blue, green, and in some cases, red or violet in an overall groundmass of gray to blue. It is thought the gray color of the groundmass is due to the scattering of light by the internal structure.

Play of Color

Another attractive feldspar mineral is adularia. Like labradorite, it develops as thin crystals that line up in parallel arrangement and act as a diffraction grating. But adularia does not show a play of color. The twinned arrangement of the crystals simply scatters light. While it can also be shades of gray, pink, peach, green and brown, it is best known for a bluish-white color that is reminiscent of the moon.

Properly cut adularia gives off a cloudy sheen that seems to float throughout the polished stone. We give this lovely form of adularia the name “moonstone.”

Why does adularia have little color, while labradorite is a riot of color? Minerals color variations are because of minor variations in the refractive index of the labradorite crystals involved. In adularia, the refractive indices of the crystals are virtually the same.

Iridescence

Iridescence is described as a play of changing colors on a surface of a mineral. A prime example is the look of oil spread over the surface of water. The oil particles have a different refractive index than the water, and this physical difference results in a play of color.

The most common example of this phenomenon is called “peacock ore”, which is actually the mineral bornite (copper sulfide). A freshly broken surface of bornite quickly oxidizes, forming a thin oxide mineral layer whose refractive index differs from bornite’s and creates a play of color. More subdued examples of this iridescence are seen on some crystal surfaces of pyrite, cuprite, chalcopyrite and hematite.

Pearl Iridescence

Iridescence is what gives pearls their soft, moonlike luster, called “orient.” Pearls are made up of layer upon layer of microscopic crystals of hexagonal aragonite. The refractive indexes of these layers are the same. Colored and black pearls result from inclusions that get into the pearl’s structure.

Mother of Pearl’s lovely shimmer, or glow, comes from the interior lining of shells, which is made up of two different substances: the calcium carbonate mineral aragonite, which forms microscopic hexagonal crystals, and conchiolin, a fibrous protein that forms in layers in parallel arrangement. The parallel fibers of the conchiolin are the key to creating the iridescence we see in mother of pearl, also called “nacre.”

Chatoyancy

When the fibers of a mineral develop in a parallel arrangement, they impart a silky shimmer or glow of light, called chatoyancy, that can be very appealing. You can expect to see this shimmer in a range of minerals. Asbestos is a very common example. When the asbestos is invaded by silica, it can form what we normally call tiger’s-eye, which is a very useful chatoyant gemstone with a silky luster. The invading silica negates the hazard we normally associate with asbestos.

Iridescence Within Stain Spar

One variety of gypsum, called stain spar, also shows iridescence, or glimmer of light. The mineral looks like silk cloth, whose fibers are also arranged in a tightly woven, parallel structure. Another example of iridescence is seen in some malachite. This copper carbonate usually crystallizes in tightly packed needles, which grow in slightly diverging radiating masses. When freshly broken, these near-parallel fibers give off a shimmering green color.

Asterism is seen in minerals like diopside, gem corundum, some moonstones, and several others. In these species, included fibrous crystals of the mineral rutile, in an intersecting arrangement, reflect light in a six-rayed star pattern. This physical phenomenon is what creates rare star sapphires and rubies, which are very valuable varieties.

Cat’s-eye gems exhibit chatoyancy, as well as a single, bright, linear reflection from tightly packed parallel fibers of a second mineral. Lapidaries give these gemstones a slight to strong dome and orient them so that the included mineral, often rutile or tourmaline, runs straight across the curved surface to form a single bright line, much like the vertical iris in a cat’s eye. It is important to know that these included needle crystals are all oriented along just one of the several growth axes of the hexagonal corundum stone.

Hexagonal Minerals

Hexagonal minerals like ruby and sapphire develop along four axes: one vertical “C” axis, from which three axes develop at right angles to the “C” axis, 60º from each other. For a star gem to form, the included mineral orients along the two arms of each horizontal axis to create a six-rayed star.

Chatoyancy is also seen in the cubic mineral gem garnet. The difference is that garnets form in the cubic system so the “star” forms from needle crystals that have oriented along the two horizontal axes that make up the cube form. Only two axes extend away from the single vertical axis, so the four arms of these axes with their parallel, included needles can orient to form a four-rayed star.

Understanding Aventurescence

Aventurescence is another physical phenomenon that involves inclusions. In this case, the inclusions are usually large enough to be visible and are scattered throughout the crystal mass, rather than oriented in a particular alignment. These scattered inclusions act as reflectors that scatter the light entering the host mineral.

An intriguing example of this is the manmade material called “goldstone”, which is glass with copper inclusions that give the glass a bright reddish-gold color.

Aventurescence is named for a quartz variety called aventurine, which is a lovely green color thanks to included chrome mica. These tiny, green flakes, or spangles, are scattered throughout the quartz, giving it a diffused green color of varying intensity that is very attractive.

The most attractive gem that falls into this category is the feldspar variety sunstone. This very lovely gem is found in several places in Oregon and shows a fine orange to red color due to included copper diffused throughout the gem. In some examples, the copper orients within the feldspar so that wisps and feathers of color are prominent in the gem. Sunstone claims in Oregon are occasionally opened to collectors for a fee.

Alexandrite Effect

Finally, the alexandrite effect is seen in very few minerals whose color is based on the type of light source. The chrysoberyl variety alexandrite is the obvious example.

Alexandrite has a light absorption band that, in sunlight, can split light into two different transmission areas. Under sunlight and fluorescent light, some of the blue wavelengths are absorbed, so green becomes dominant. When seen under in incandescent light, alexandrite is red.

As you collect colorful minerals, be aware that not all of them owe their color to a trace element inclusion. This is another area of interest you can pursue as you enjoy our wonderful hobby.

This story about what gives minerals color appeared in Rock & Gem magazine. Click here to subscribe. Story by Bob Jones.

The post What Gives Minerals Color? first appeared on Rock & Gem Magazine.

The word “iridescence” stems from the Latin words iris, or “rainbow,” and descendere, or “coming down.” Its literal meaning, “from the rainbow,” fits an optical phenomenon that manifests itself as a rainbow-like play of vivid colors.

Nature of Iridescence

While iridescence occurs throughout the natural world, notably in certain bird feathers, flower petals, reptile scales, and seashells, it is particularly apparent in minerals. Iridescence can be a diagnostic property in chalcopyrite, bornite, covellite, and other metal sulfides. It can increase the value of mineral specimens and is the basis of the visual appeal of such gem materials as opal, labradorite, and fossil “ammolite.”

Iridescence differs from other optical phenomena in the nature of its colors. Most colors are spectral or pigmentary in origin and are produced, respectively, by the diffraction and absorption of white light. But iridescent colors, also called structural colors, have a structural origin and are created by reinforcing reflected light waves.

Light and Iridescence

Light is a form of electromagnetic radiation that is detectable by the human eye. Light exhibits properties of both waves and particles and travels in waves that are measured by frequency (wavelengths), phase (positions of wave crests and troughs), and amplitude (wave magnitude).

The wavelengths of visible light extend from 380 nanometers (red) to 760 nanometers (violet). “White” light, a mix of all visible wavelengths, can be diffracted or separated into its spectral color components of red, orange, yellow, green, blue, indigo, and violet.

Examples of Spectral Colors

Examples of spectral colors are the hues of rainbows and the spectrum created when a prism diffracts light. Pigmentary colors are produced when materials absorb specific wavelengths of white light and reflect others. Objects that we perceive as red absorb the orange, yellow, green, blue, indigo, and violet wavelengths and reflect only the red wavelengths.

Structural colors have an entirely different origin and are created when certain microscopic structures modify light waves, reflecting and becoming reinforced.

Peacock feathers are textbook examples of how pigmentary colors and structural colors differ. The pigmentary color of peacock feathers is dull brown, but the interaction of light with microstructures on the feather surfaces creates the intense, green-and-blue structural colors of iridescence.

Understanding Iridescence

The cause of iridescence was a scientific puzzle for centuries. In 1665, English naturalist Robert Hooke noted that the “fantastical” iridescent colors of peacock feathers were entirely unrelated to their brown pigmentary color. A half-century later, English mathematician and physicist Sir Isaac Newton, a proponent of the particle theory of light, refined Hooke’s idea by concluding that peacock-feather surfaces somehow modified light to cause iridescence.

In 1804, English physicist Thomas Young, who believed that light behaved as a wave, observed that peacock-feather surfaces consisted of repetitive, microscopic ridges that diffracted light into its spectral components. After reflecting, these components seemed to interact to gain brilliance and intensity. In 1892, English zoologist Frank Evers Beddard coined the term “structural colors” to describe iridescence’s vivid color.

Optical Interference and Iridescence

By 1900, scientists understood that iridescent structural colors resulted from the phenomenon of optical interference. Optical interference occurs when two or more reflected light waves of the same frequency and traveling in parallel paths overlap in coordinated phases.

Optical interference can be destructive or constructive. In destructive interference, out-of-phase wavelengths cancel each other out. In constructive interference, the cause of iridescence, the overlapping waves are in phase and reinforce each other.

Reinforcement increases wave amplitude to create structural purple, cyan, green, metallic copper and gold, and magenta colors. Because constructive interference reinforces only a single wavelength at a time, the resulting structural color has greater vibrancy than the corresponding spectral or pigmentary colors and quality described as “pure,” “electric,” “vibrant,” and “neon-like.”

Thin-Film Interference

Two different types of microscopic structures produce optical interference: thin films and diffraction gratings. In thin-film interference, light strikes a surface consisting of two superposed, parallel, reflective planes separated by a distance roughly equal to the wavelengths of light.

The upper layer reflects some of the incident light and transmits the remainder to the underlying layer. These planes then reflect two slightly offset beams traveling in the same direction. Reinforcement occurs when specific wavelengths within these beams are in a coordinated phase.

Diffraction-Grating Interference

Unlike thin-film interference, diffraction-grating interference requires a single, reflective surface composed of periodic, parallel, nanoscale ridges or grooves separated in distance by the wavelengths of light.

The edges of these ridges diffract light into its wavelength components; reinforcement occurs when the reflected beams are in a coordinated phase. Despite their different structural origins, the iridescence produced by both thin-film and diffraction-grating interference is identical.

Iridescent Sulfide Minerals

Iridescence can appear in a surprisingly large number of minerals with differing chemistries, crystal structures, transparency and luster.

It is most common in opaque metal sulfides with a metallic luster. These include chalcopyrite [copper iron sulfide, CuFeS2], bornite [copper iron sulfide, Cu5FeS4], covellite [copper sulfide, CuS], stibnite [antimony sulfide, Sb2S3], bismuthinite [bismuth sulfide, Bi2S3], pyrite [iron disulfide, FeS2], and the skudderite series of cobalt-nickel arsenides.

The surfaces oxidize or tarnish into microscopically thin films of metal oxides. In chalcopyrite, surface oxidation creates a film of hematite [iron oxide, Fe2O3], goethite [basic iron oxide, FeO(OH)], and limonite (a mix of ferric and ferrous oxides and hydroxides).

Repetitive oxidation phases and subsequent weathering can create multiple, superposed oxide layers. When properly spaced, these layers cause thin-film interference.

Not Always Iridescent

Chalcopyrite and other metal sulfides are not always iridescent. Iridescence occurs only in specimens in which the distance between the surface oxide films approximates that of the wavelengths of light. Variations within this distance determine the intensity and specific wavelengths of the resulting structural colors.

Rotating a specimen of chalcopyrite changes the incident-light angle and thus the distance that light travels between the two reflective surfaces. This movement varies the frequencies of the reinforced wavelengths to create kaleidoscopic displays of continuously changing colors.

Structural Color Ranges

Many metal sulfides tend to exhibit specific structural-color ranges. Chalcopyrite’s iridescent colors usually consist of gold and magenta, while bornite’s lean toward gold and cyan with only occasional flashes of magenta.

In covellite and stibnite, the distancing of the respective copper-oxide and tin-oxide layers produces a cyan or purple iridescence. The layer separation in polybasite (a complex silver copper antimony arsenic sulfide) creates a predominantly green iridescence.

Surface & Internal Iridescence

While opaque metal sulfides show only surface iridescence, translucent or transparent minerals can display surface and internal iridescence. Among these is the labradorite variety of the feldspar-mineral anorthite [calcium aluminum silicate, CaAl2Si2O8].

The internal layers of labradorite are twinned, translucent microcrystals create both thin-film and diffraction-grating interference and a distinctive type of iridescence called labradorescence, a multicolored, subsurface sheen that sweeps broadly across the entire stone. Most labradorite exhibits cyan, green, and gold structural colors with only hints of magenta. The intensity of labradorescence varies greatly among individual specimens.

Labradorite’s History

Labradorite’s first reference is on the Isle of Paul in today’s Canadian province of Newfoundland and Labrador. The Isle of Paul’s granite bedrock consists mostly of anorthite, which, at certain angles of sunlight, glows with labradorescence.

The native Inuit people likened its eerily shifting, structural colors to those of the northern lights. Cut-and-polished labradorite often appears in pendants. Specimens with particularly intense labradorescence are known in the jewelry trade as “spectrolite.” The national gemstone of Finland is a type of spectrolite that is unusually rich in gold and magenta.

Several commercial architectural stones, such as “blue granite” and “labradorite granite,” contain evenly dispersed labradorite phenocrysts that create a subtle but attractive labradorescence.

This story about iridescent minerals is part of a two-part series that previously appeared in Rock & Gem magazine. Click here to subscribe. Story by Steve Voynick.

The post The Essence of Iridescent Minerals first appeared on Rock & Gem Magazine.

Labradorite was first discovered on Paul’s Island (Paul Island, Isle of Paul) in Labrador, Canada, which is its namesake. Although discovered in Canada, this mineral can be found in several other places throughout Scandinavia.

Labradorite is a feldspar with a very unique iridescent schiller effect, with a wide range of colors ranging from blue, green, red, orange and yellow, giving it an outstanding play of color known as labradorescence.

Colors Draw Attention

Labradorite is closely related to other feldspar materials with similar schiller effects such as spectrolite, moonstone, larvikite, and even sunstone. However, labradorite has become one of the most well known and well-used stone in lapidary due to its spectacular display of colors.

Buying labradorite can be tricky. Though it’s simple to find in the marketplace, it can be very difficult to choose a rough stone just by its outer appearance. Even when you can see color flashes on the outside, it doesn’t mean it will continually run through the entire stone. In fact, most of the time the color plains will constantly change direction, making it a very difficult stone to slab. Labradorite can be found in large size pieces, but unless you really know what you are looking for,

I would highly suggest buying several smaller size chunks, allowing you the freedom to hand cut on a smaller size trim saw.

By having the ability to hand cut, you have a better chance of chasing the color flashes as they continue to change direction.

The first thing to do before cutting slabs is wet your material and hold it vertically with a light source behind you. Continue to rotate the stone in every direction until you find the area that gives you the best color flash display, and that’s where you start to cut your first slab from. With each cut, you will see if the color flash has run its course or not. You might be able to cut a few slabs continuing in that direction, or the color may have dithered out and you will once again have to put it up to the light to determine the direction.

Rotate the Slab To Find ‘Flash’

Once you have your slabs cut and you are ready to design your preforms, its time to go back to the light source once again. Hold your slab vertically and continue to rotate it until you see the best flash. Visualize it as if it were a pendant worn by someone standing in front of you. Once you see the flash, mark the areas with the desired design of cabs you wish to cut, and you’re ready to bring it back to the trim saw and cut.

Slabs should be no more than 1/4” thick, because once you start to grind your cabs, the higher the dome, the more chance you’ll grind some of the flash away and end up with a semi-translucent grey area. With that in mind, be careful you don’t cut the slabs too thick. Plus, keep your cab domes medium to low height for optimal flash effects.

Cabbing labradorite is fairly easy, treat it as you would any jasper. Start at the 80 grit and work your way across all the wheels. It cuts nice and evenly and is fairly easy to remove scratches along the way. However, once you finish up, usually on the 14k grit wheel, it will still have a semi-gloss luster finish.This is one of those stones that really benefits from polishing with cerium oxide on a leather wheel and buffing it out to get that nice shiny mirror polish. Be sure when you do, to go in slow stages and continue to check the cab and be sure not to overheat it, or it might start to crack.

Once you are finished, have a damp towel handy to wipe off any excess polishing compounds, and you should have a beautiful mirror polished labradorite cab just bursting with color!

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