Watch a piece of tanzanite rotate under polarized light, and you’ll witness one of nature’s most captivating optical displays—violet shifting to blue, then to rich brown, all from the same crystal. This remarkable phenomenon, known as pleochroism, serves as one of the most reliable methods for identifying crystals and understanding their internal atomic structure.
Gemstone pleochroism is the phenomenon where certain gemstones display multiple colours when viewed from different angles, and it plays a significant role in gemstone identification.
Pleochroism represents a fundamental optical property that distinguishes anisotropic crystals from their isotropic counterparts, making it an invaluable tool for mineralogists, gemologists, and collectors worldwide. The International Gem Society recognizes pleochroism as a critical diagnostic criterion in gemstone identification, helping distinguish natural stones from synthetic alternatives and revealing the crystalline structure that determines a mineral’s unique properties.
In this comprehensive guide, we’ll explore how polarized light interacts with different crystal systems, examine notable pleochroic minerals that exhibit this phenomenon, and discuss practical methods for detecting and measuring these remarkable colour changes.
What is Pleochroism
Pleochroism is an optical phenomenon where anisotropic crystals display different colours when viewed from different angles under polarized light. The term originates from the Greek words “pleio,” meaning “more,” and “chroma,” meaning “colour,” and perfectly describes this multi-coloured display that reveals the internal organization of crystal structures.
This fascinating property arises from the crystal’s internal atomic structure, which affects how light is absorbed and transmitted along different crystallographic axes. The number of colours visible from different angles is determined by the crystal structure, which influences how light interacts with the mineral. When plane-polarized light enters an anisotropic crystal, the electronic structure selectively absorbs various wavelengths depending on the direction of light propagation through the crystal lattice.
The phenomenon proves crucial for mineral and gemstone identification because each species exhibits characteristic pleochroic colours that remain consistent regardless of specimen size or origin—pleochroism results in more colours being observed in certain minerals, which aids in their identification. Gemologists routinely use pleochroism to identify crystals, authenticate gemstones, and distinguish between minerals with similar physical appearances but distinct optical properties.
Unlike simple colour zoning or surface effects, pleochroism reflects the fundamental relationship between crystal symmetry and optical properties. Pleochroism results from the interaction of light with the crystal's structure and orientation. This makes it particularly valuable when other identification methods prove inconclusive or when examining small mineral grains in thin sections under a petrographic microscope. When introducing dichroism, it is important to note that it involves the display of two colours in uniaxial crystals.
The Science Behind Pleochroism
The science underlying pleochroism begins with understanding how polarized light interacts with anisotropic crystal structures. When ordinary light—which vibrates in multiple directions—encounters an anisotropic crystal, the crystal’s internal structure splits this light into component rays that vibrate in specific directions relative to the crystal’s optical axes. The role of optic axes is crucial, as they determine the directions in which light can propagate through the crystal and influence the resulting optical effects.
Double refraction underlies all pleochroic effects. As incident light enters an anisotropic crystal, it splits into two rays travelling at different speeds through the crystal. These rays, polarized at right angles to each other, follow different paths and experience varying levels of absorption depending on their interaction with the crystal’s electronic structure. The different polarizations of light interact uniquely with the crystal structure, resulting in distinct optical behaviours such as dichroism and birefringence.
The varying refractive indices in different crystal directions cause colour changes as light vibrates along distinct crystallographic axes. Each direction through the crystal has a unique refractive index, creating an environment in which specific wavelengths of light are absorbed differently. In one direction, light may be absorbed more strongly, leading to noticeable colour changes. This selective absorption produces the characteristic colours visible when pleochroic minerals are rotated under polarized light. In uniaxial minerals, this can result in the display of two pleochroic colours depending on the viewing angle.
Anisotropic crystals demonstrate this behaviour because their atomic structure lacks the symmetrical arrangement found in isotropic materials. Glass and other isotropic materials show uniform light transmission in all directions, never displaying true pleochroism because their internal structure treats light equally regardless of direction.
The absorption spectrum varies significantly along different optical axes, with chromophore ions and molecular bonds within the crystal lattice determining which wavelengths are absorbed or transmitted. This creates the foundation for using pleochroic properties as a diagnostic tool in mineral identification.
Types of Pleochroism
Pleochroism manifests in two primary forms, determined by the number of optical axes present in the crystal structure and the resulting colour variations observable under polarized light.
Dichroism, which causes crystals to show two distinct colours, occurs in uniaxial crystals with one optical axis. These crystals, belonging to hexagonal, tetragonal, and trigonal crystal systems, split light into ordinary and extraordinary rays that experience different absorption along their paths through the crystal structure.
Tourmaline provides an excellent example of dichroism, displaying dramatic colour contrasts when rotated under polarized light. Green tourmaline specimens often show bright green colours when viewed along one axis and nearly black or dark green when rotated 90 degrees. Pink tourmaline similarly shifts from vibrant pink to almost colourless depending on the viewing angle.
Emerald and sapphire also exhibit dichroism, though it is often more subtle than in tourmaline. Emerald typically shows blue-green to green colour variations, while sapphire can display blue to greenish or even yellow tones depending on the crystalline structure orientation.
Trichroism is the most complex form of pleochroism, occurring in crystals that display three colours. Biaxial minerals with two optical axes demonstrate this phenomenon, primarily found in orthorhombic, monoclinic, and triclinic crystal systems, where light splits along multiple directions.
Andalusite demonstrates pronounced pleochroism with distinct colour changes under polarized light, showing green, brown, and yellow tones along its three principal optical axes. This strong pleochroism makes andalusite easily identifiable even in small specimens or thin sections.
Cordierite, commonly known as iolite, exhibits exceptional trichroism that historically served practical purposes. This mineral displays violet-blue and yellow-gray colour axes so distinctly that Vikings allegedly used iolite crystals as primitive polarizing filters for navigation, detecting the sun’s position even through cloud cover.
Tanzanite represents perhaps the most spectacular example of trichroism, displaying violet, blue, and brown colours in different directions. This remarkable colour play contributes significantly to tanzanite’s value as a gemstone, with gem cutters carefully orienting facets to maximize the most desirable blue and violet hues.
Crystal Systems and Pleochroism
The relationship between crystal symmetry and optical properties determines which minerals can exhibit pleochroism and how many colours they might display when examined under appropriate conditions.
Uniaxial systems encompass the hexagonal, tetragonal, and trigonal crystal classes, which exhibit up to two pleochroic colours due to their single optical axis. Light entering these crystals splits into ordinary and extraordinary rays with different vibration directions, creating dichroic effects.
Most crystals in uniaxial minerals demonstrate this behaviour consistently, with the ordinary ray following Snell’s law while the extraordinary ray experiences direction-dependent refractive indices. This fundamental difference in light behaviour produces the two distinct colours observable in dichroic minerals.
Biaxial systems include orthorhombic, monoclinic, and triclinic crystal classes, which can display up to three pleochroic colours due to their two optical axes configuration. The more complex internal structure creates additional opportunities for light to interact differently along various crystallographic directions.
Biaxial minerals possess three principal refractive indices, each associated with distinct absorption characteristics. This arrangement allows for trichroism, in which rotating the crystal reveals three distinct colour axes corresponding to the three principal vibration directions within the crystal structure.
Isotropic systems, specifically the cubic crystal system, never show true pleochroism because their highly symmetrical internal arrangement treats light equally in all directions. Minerals crystallizing in the cubic system, such as garnet and fluorite, maintain consistent optical properties regardless of viewing angle.
Crystal symmetry directly influences whether a mineral exhibits pleochroism and determines the maximum number of colours visible. Understanding these relationships helps identify crystals and predict their optical behaviour before examination under polarized light.
This systematic approach allows mineralogists to categorize pleochroic minerals by crystal system and to use this information as an additional identification criterion when examining unknown specimens.
Observing Crystals
Observing crystals under polarized light unlocks a world of hidden colour and structure, offering a direct window into the optical properties that make each mineral unique. When a crystal is placed beneath a microscope equipped with a polarizing filter, it can exhibit pleochroism—changing colour as the direction of incident light or the crystal's orientation changes. This captivating effect is a direct result of the crystal’s anisotropic nature, in which the internal atomic arrangement causes light to vibrate at different speeds and in different directions.
The process begins with the use of plane polarized light, which vibrates in a single direction. By passing this light through a crystal, observers can identify the crystal’s optical axes and measure its refractive indices—key indicators of how much the crystal bends and absorbs light in various directions. In anisotropic crystals, the refractive index is not uniform; it varies with the direction of light travel, resulting in pleochroic colours that range from blue and green to brown and beyond.
A common technique for revealing these optical properties involves preparing thin sections of crystals. These ultra-thin slices allow light to pass through the mineral, making even subtle pleochroic effects visible. Placing a thin section on the stage of a petrographic microscope—an instrument designed for mineral analysis—enables the observer to rotate the sample and watch as the pleochroic colours shift with each turn. This rotation is crucial: as the microscope stage moves, the direction of light relative to the crystal’s structure changes, causing the colours to vary and revealing the mineral’s possible crystalline structure.
The International Gem Society recognizes the importance of pleochroism in gemstone identification, and observing pleochroic properties is a standard practice among gemologists. For example, tanzanite is renowned for its strong pleochroism, which displays vibrant blue, green, and brown hues depending on the direction of light. By carefully analyzing these colour changes, experts can not only identify the gemstone but also assess its quality and value.
Beyond gemstones, pleochroism is a valuable diagnostic tool in mineralogy. When analyzing mineral grains, especially in rock thin sections, polarized light helps reveal their pleochroism, which indicates a mineral’s crystalline structure and composition. The colours observed—whether subtle or dramatic—offer clues about the mineral’s electronic structure and the way light interacts with its atomic framework.
In summary, observing crystals under polarized light—using plane polarized light, thin sections, and petrographic microscopes—provides a powerful method for uncovering the pleochroic colours and optical axes that define a mineral’s identity. Whether you’re identifying a rare gemstone or studying the structure of a mineral grain, these techniques illuminate the intricate relationship between light, colour, and crystalline structure, deepening our appreciation for the natural beauty and complexity of the mineral world.
Notable Pleochroic Minerals and Gemstones
Strong Pleochroism Examples
Certain minerals exhibit such pronounced pleochroism that the colour changes are visible even without specialized equipment, making them excellent examples of this optical phenomenon.
Tourmaline stands among the most dramatic examples of pleochroic minerals, showing bright green versus black or pink versus colourless contrasts depending on the viewing direction. Green tourmaline specimens often appear nearly transparent when viewed along the crystal’s length but show rich, saturated green when viewed from the side. This extreme dichroism affects how gem cutters orient tourmaline stones to maximize colour saturation.
Pink tourmaline demonstrates equally striking pleochroism, shifting from vibrant rose pink to nearly colourless as the crystal rotates. This dramatic colour variation makes tourmaline an ideal mineral for demonstrating pleochroic principles to students and collectors.
Tanzanite displays exceptional trichroism, revealing violet, blue, and brown colours in different directions with remarkable clarity. The brown axis is typically considered the least desirable to gemstone enthusiasts, leading lapidaries to orient cuts to minimize brown tones while emphasizing the prized blue and violet hues.
The strong pleochroism in tanzanite directly impacts its commercial value, as stones showing muddy or mixed colours command lower prices than those displaying clean, saturated blues and violets. This economic factor demonstrates how pleochroic properties influence both scientific classification and commercial applications.
Cordierite (Iolite) exhibits distinctive violet-blue and yellow-gray colour axes that create one of the most recognizable pleochroic signatures in the mineral kingdom. The contrast between these colours is so pronounced that iolite serves as an excellent teaching specimen for demonstrating trichroism to beginners.
Historical accounts suggest Vikings used iolite’s pleochroic properties for navigation, though this remains a subject of ongoing research. The mineral’s ability to reveal polarized skylight patterns could theoretically help locate the sun’s position even under overcast conditions.
Andalusite demonstrates distinct colour changes under polarized light, typically showing green, red, and yellow along its three principal axes. The colours often appear simultaneously in properly oriented specimens, creating a unique visual effect that makes andalusite unmistakable when properly illuminated.
Moderate to Weak Pleochroism Examples
Not all pleochroic minerals display dramatic colour changes; many show subtle but detectable variations that require careful observation or specialized equipment to appreciate fully. The pleochroic effects observed in minerals can vary widely, from dramatic to subtle, depending on the mineral and its crystal structure.
Epidote shows green-to-amber-orange colour shifts, exemplifying moderate pleochroism. While less dramatic than tourmaline or tanzanite, epidote’s colour variations remain consistent and useful for identification purposes, particularly when examining thin sections under a petrographic microscope.
The pleochroic colours in epidote often appear more pronounced in transmitted light than in reflected light, making polarizing microscopes essential for proper observation. This characteristic demonstrates why gemologists and mineralogists rely on standardized lighting conditions when evaluating pleochroic properties.
Kyanite displays subtle colour variations along its crystal axes, which require careful observation to detect. The pleochroism typically manifests as slight shifts in blue intensity rather than complete colour changes, making kyanite an example of weak but measurable pleochroism.
Despite its subtle nature, kyanite’s pleochroism remains diagnostically useful when combined with other optical properties. The consistent pattern of colour variation helps distinguish kyanite from similar-appearing minerals in field and laboratory settings.
Amethyst and other quartz varieties exhibit subtle but observable pleochroism, surprising many collectors who assume quartz lacks significant optical complexity. Purple amethyst typically shows slight variations between violet and reddish-purple depending on the viewing angle.
Titanite shows high birefringence and distinct pleochroic colours, typically ranging from yellow to brown to green. The combination of strong birefringence and clear pleochroism makes titanite easily identifiable under polarized light, despite its relative rarity in most collections.
Detection and Measurement Methods
Observing and measuring pleochroism requires specific equipment and techniques designed to control light polarization and viewing angles, enabling accurate assessment of colour variations in crystalline materials.
Dichroscope serves as the most common handheld tool for observing transmitted light colours at different angles in gemstones and mineral specimens. This simple instrument contains polarizing filters arranged to reveal pleochroic colours simultaneously, allowing direct comparison of different colour axes.
Using a dichroscope effectively requires proper lighting and specimen orientation. Natural daylight or full-spectrum artificial lighting provides the best results, while rotating the specimen reveals the full range of pleochroic colours available in each mineral. The International Gem Society recommends dichroscopes as essential equipment for gemstone identification.
Polarizing microscope (petrographic microscope) offers the most sophisticated method for examining thin sections under polarized light. This instrument provides precise control over light polarization and allows detailed analysis of pleochroic properties in rock thin sections and prepared mineral specimens.
Petrographic microscopes enable quantitative assessment of pleochroism by measuring colour intensity variations along specific crystal axes. The rotating microscope stage enables systematic examination of pleochroic properties while maintaining consistent lighting conditions throughout the observation.
Polariscope represents a specialized gemological instrument for detecting optical properties in transparent gemstones. While primarily designed for identifying double refraction, polariscopes can reveal pleochroic colours in properly oriented specimens under controlled lighting conditions.
Visual observation techniques using polarizing filters provide accessible methods for detecting pleochroism without expensive equipment. Simple polarizing filters, available from photography suppliers, can reveal pleochroic effects when used with proper lighting and specimen rotation.
Successful visual observation requires understanding how to orient specimens relative to polarizing filters and light sources. Rotating specimens while observing through polarizing filters reveals colour changes that indicate pleochroic properties, though this method works best with strongly pleochroic minerals.
The key to accurate pleochroism detection lies in standardizing observation conditions and understanding how different lighting affects colour perception. Professional gemologists and mineralogists establish consistent protocols for examining pleochroism to ensure reliable identification.
Applications in Mineralogy and Gemology
Pleochroism serves multiple practical purposes in both scientific research and commercial applications, making it an indispensable tool for professionals working with crystalline materials.
Mineral identification relies heavily on revealing crystalline structure through colour patterns that remain consistent across specimens of the same species. When examining unknown minerals, pleochroic properties often provide definitive identification when other physical characteristics prove ambiguous.
Field geologists use portable dichroscopes to identify crystals in outcrop samples, particularly in fine-grained rocks where individual crystals are difficult to examine. The ability to detect pleochroism quickly helps distinguish between minerals with similar appearances but different crystalline structures.
Gemstone authentication depends partly on pleochroic properties to distinguish natural from synthetic materials. Many synthetic gemstones lack the subtle pleochroic variations found in their natural counterparts, making pleochroism a valuable authentication tool.
Natural tanzanite, for example, exhibits characteristic trichroism that differs from that of synthetic alternatives. Similarly, natural tourmaline exhibits pleochroism, which helps gemologists identify treated or synthetic stones that attempt to imitate natural varieties.
Quality assessment in gem cutting requires understanding pleochroic properties to optimize colour display in finished stones. Skilled lapidaries study rough gemstones under polarized light to determine optimal cutting orientations that emphasize desirable colours while minimizing less attractive pleochroic directions.
The economic impact of proper orientation can be substantial for valuable gemstones such as tanzanite and high-quality tourmaline. Understanding pleochroic properties allows gem cutters to maximize both beauty and value in their finished products.
Differentiating minerals with similar appearances but distinct pleochroic schemes provides another crucial application in both field and laboratory settings. Minerals that appear nearly identical in hand specimen often reveal distinctive pleochroic signatures under polarized light.
This application is particularly valuable when examining metamorphic rocks that contain multiple minerals with similar colours and crystal habits. Pleochroic properties help distinguish between species that might otherwise require expensive analytical techniques for identification.
Research applications include using pleochroic halos—coloured spherical shells surrounding radioactive inclusions—to study geological history and the distribution of radioactive elements in rocks. These microscopic features provide insights into the thermal and chemical evolution of rock formations over geological time.
Historical and Cultural Significance
The discovery and application of pleochroism spans centuries of scientific advancement and cultural adaptation, demonstrating how optical phenomena influenced both navigation technology and modern analytical methods.
Viking “sun stone” navigation represents one of the most fascinating historical applications of pleochroic properties. Archaeological and experimental evidence suggests that Vikings used iolite’s pleochroic properties to locate the sun through cloud cover during ocean voyages.
The technique allegedly involved rotating iolite crystals while observing polarization patterns in skylight invisible to the naked eye. When the crystal showed specific colour combinations, navigators could determine the sun’s approximate position even when direct visibility was impossible. Modern experiments have demonstrated the theoretical feasibility of this navigation method.
While debate continues regarding the extent of Viking navigation using pleochroic minerals, the concept illustrates how ancient peoples observed and utilized optical phenomena without understanding the underlying scientific principles.
Development of polarizing microscopy in the 19th century revolutionized mineral studies and established pleochroism as a systematic identification tool. Early petrographers recognized that rotating mineral specimens under polarized light revealed consistent colour patterns that correlated with crystal structure.
This development transformed geology from a primarily descriptive science into a more analytical discipline capable of identifying minerals with precision. The ability to study rock thin sections under polarized light enabled geologists to determine rock compositions and formation histories with unprecedented accuracy.
Auguste Michel-Lévy’s contributions to interference colour charts and polarization studies established standardized methods for describing and measuring optical properties in minerals. His systematic approach to cataloguing pleochroic properties provided the foundation for modern petrographic techniques.
Michel-Lévy’s work demonstrated that quantitative measurement of optical properties could reveal details of crystal structure invisible to other 19th-century analytical methods. This laid the groundwork for modern crystallographic studies that continue to rely on optical property measurements.
The evolution from geological petrography to modern gemological applications shows how scientific understanding of pleochroism expanded from pure research into commercial applications. Today’s gemological instruments directly descend from 19th-century petrographic microscopes, adapted for examining precious stones rather than rock specimens.
Practical Considerations for Collectors
Understanding pleochroism enhances both the scientific appreciation and practical enjoyment of mineral collecting, though successful observation requires attention to specific techniques and equipment considerations.
Proper lighting conditions are essential for accurately observing pleochroism. Natural daylight provides the most reliable illumination for detecting colour changes, though full-spectrum artificial lighting can substitute when natural light is unavailable.
Avoid fluorescent or LED lighting with limited spectral ranges, as these can mask subtle colour variations or create artificial colour effects that complicate the observation of pleochroism. Incandescent lighting, while warmer than ideal, often reveals pleochroic colours better than fluorescent alternatives.
Distinguishing pleochroism from colour-change phenomena requires understanding the fundamental differences between these optical effects. Pleochroism depends on crystal orientation relative to polarized light, while colour change phenomena, like that seen in alexandrite, result from different spectral compositions of incident light.
Colour zoning—where different parts of the same crystal show different colours due to compositional variations—can also be confused with pleochroism. True pleochroism affects the entire crystal uniformly, with colour changes occurring only as the crystal rotates relative to the observer.
Accessibility of thin sections versus large specimens presents practical considerations for amateur mineral enthusiasts. While thin sections provide the clearest pleochroic displays under microscopic examination, they require expensive preparation and specialized equipment for proper observation.
Large, well-formed crystals often show pleochroism to the naked eye when properly oriented under polarized light. Collectors should focus on acquiring specimens with good crystal faces and minimal inclusions for the best results when studying pleochroic properties.
Equipment recommendations for hobbyist mineral identification should emphasize versatility and cost-effectiveness. A quality dichroscope is the most practical investment for serious collectors, providing reliable pleochroism detection without the expense of a microscope.
Polarizing filters from photography suppliers offer an economical alternative for basic pleochroism detection. When combined with proper lighting and systematic specimen rotation, these simple tools can reveal pleochroic properties in strongly pleochroic minerals.
Building a reference collection of known pleochroic minerals helps develop the observational skills needed to identify unknown specimens. Start with strongly pleochroic examples like tourmaline and iolite before progressing to minerals with subtler colour variations.
Document observations systematically, noting lighting conditions, viewing angles, and colour descriptions for future reference. This practice establishes a consistent methodology for reliable mineral identification based on pleochroic properties.
Consider joining local mineral clubs or geological societies where experienced collectors can demonstrate proper techniques for observing pleochroism. Hands-on instruction often proves more effective than written descriptions for mastering the subtle skills required for optical property assessment.
Conclusion
Pleochroism represents one of nature’s most elegant demonstrations of the relationship between crystal structure and optical properties, offering both scientific insight and practical applications across multiple disciplines. From its role in mineral identification to its influence on gemstone valuation, this optical phenomenon remains a fundamental tool for understanding crystalline materials.
The journey from ancient navigation techniques to modern gemological applications illustrates how systematic observation of natural phenomena leads to both scientific advancement and practical innovation. Today’s sophisticated analytical instruments trace their origins to simple observations of colour changes in rotating crystals under polarized light.
For collectors, students, and professionals alike, mastering pleochroism observation opens new dimensions of appreciation for mineral specimens while providing reliable identification methods that complement other analytical techniques. Whether examining rare gemstones or common rock-forming minerals, understanding pleochroic properties enhances both scientific knowledge and aesthetic enjoyment.
As analytical techniques continue advancing, pleochroism remains relevant for authentication, identification, and quality assessment in an era of increasing synthetic alternatives and sophisticated treatments. The fundamental relationship between crystal symmetry and optical properties ensures that pleochroism will continue serving as a diagnostic tool regardless of technological developments.
Begin exploring pleochroism with strongly pleochroic minerals like tourmaline or iolite, using proper lighting and simple polarizing filters to observe the remarkable colour changes that reveal the hidden structure within crystalline materials.
Frequently Asked Questions about Pleochroism
What is an example of pleochroism?
A classic example is tanzanite, which can display blue, violet, and burgundy tones depending on the viewing angle. Iolite, known as the “Viking’s Compass,” is another excellent pleochroic stone, shifting between blue-violet, grey, and honey-yellow.
What do you mean by pleochroism?
Pleochroism is a visual effect in certain gemstones where the stone appears to show different colours when viewed from different angles. This happens because light travels through the crystal structure differently along each crystallographic direction, absorbing or transmitting different wavelengths.
Which type of gem can show pleochroism?
Only anisotropic gemstones—those whose atomic structure interacts with light differently depending on direction—can show pleochroism. This includes stones from the tetragonal, hexagonal, orthorhombic, monoclinic, and triclinic systems—examples: tanzanite, iolite, kunzite, andalusite, tourmaline, sapphire, and cordierite.
What is the difference between pleochroism and dichroism?
Pleochroism is the general term for the multiple colours a gem displays when viewed from different angles.
Dichroism is a specific type of pleochroism in which a gem shows two colours.
Trichroism refers to three colours.
So pleochroism is the umbrella effect; dichroism and trichroism are its subtypes.
What gem could never be pleochroic?
Isotropic gemstones can never display pleochroism because their crystal structure responds to light equally from all directions. This includes diamonds, garnets, spinels, and glass.
How does pleochroism affect gem quality?
Pleochroism can enhance or diminish a stone's quality, depending on the stone and how it’s cut. A skilled cutter will orient the gem to highlight the most desirable hue. In stones like tanzanite or kunzite, strong pleochroism can improve beauty and value. In others, unwanted or uneven pleochroic tones may reduce saturation or create patchiness.
Which gem can show three pleochroic colors?
Tanzanite, iolite, andalusite, and some varieties of kunzite can all be trichroic, showing three distinct colours depending on the viewing direction.
Are sapphires pleochroic?
Yes. Sapphires are pleochroic, typically showing two colours (dichroic), such as blue and greenish-blue or blue and violet, depending on the variety. The effect is usually subtle but becomes more noticeable in lighter or less saturated stones.
