The earth consists of elements made up of countless atoms. Most of these atoms are in orderly, solid arrangements. Materials with such orderly atomic arrangements are said to be crystalline and each different atomic arrangement is called a crystal structure (a regular, repeating, three-dimensional arrangement of bonded atoms).
Most gems are crystalline (i.e., they possess a crystal structure). Atoms bond together most efficiently as orderly crystal structures and as a result, automatically try to pack into the most orderly structure possible. Crystalline materials can be either:
Aggregates: These are composed of crystal groups too small to be seen with the naked eye. Microcrystalline aggregates are those where individual crystals are visible with a normal microscope (e.g., aventurine quartz).Cryptocrystalline aggregates are those with a structure so fine that no distinct grains are recognizable under the microscope (e.g., chalcedony quartz).
Some gems exhibit unusual optical effects known as phenomena. These rare and beautiful effects often add value to gems. The cat’s eye effect, the star effect and the color change effect are very popular phenomena and are highly coveted. It is the possession of different crystal structures offered by many gemstones that gives them their unique properties. Without these unique properties, our ancestors would have never valued them as unusual, rare, desirable or beautiful and, in turn, without them, our gemstone choices would be very limited.
Moonstone shows a blue-whitish opalescence (sometimes described as a billowy light and shimmer) that glides over the surface of the gem. Interference phenomena from the gem’s layered structure are the cause of this effect.
Also known as the star effect, this is a reflection effect that appears as two or more intersecting bands of light across the surface of a gem. It is usually created through reflection of light by thin fibrous or needle-like inclusions (termed ‘silk’) that lie in various directions. There are 6-ray, 4-ray and, rarely, 12-ray stars. Ruby and sapphire cabochons can sometimes very effectively show this phenomenon. Gemstones will produce this effect when:
Asterism (and chatoyancy) is most visible with direct light, such as a fiber optic light, penlight or another single beam of light, including direct sunlight. With diffused illumination, stars and cat’s eyes are not as distinct (often a problem under TV studio lights).
This is a colorful play of glittering reflections of small, plate or leaf-like metallic inclusions. Gemstones that display this phenomenon include aventurine (after which the phenomenon is named) and sunstone.
Also known as the cat’s eye effect, this is a reflection effect that appears as a single bright band of light across the surface of a gemstone, similar to the slit eye of a cat. It is caused by the reflection of light by parallel fibers, needles or channels. Gemstones will show a chatoyant reflection when:
One of the most coveted cat’s eye gems is cat’s eye chrysoberyl—so much so, that if you just mention cat’s eye, it is assumed to be in reference to cat’s eye chrysoberyl. All other cat’s eye gems, such as those found in tourmaline or tiger’s eye, typically have an additional designation.
The color change or alexandrite effect is a rare, beautiful and desirable property in gemstones. White light is made up of a balanced mixture of red, orange, yellow, green, blue and violet light. When white light enters a gem, the stone absorbs some spectral wavelengths more than others, a process termed selective absorption. Since the unabsorbed light exiting the stone is no longer in balance, the object appears colored.
But all “white” light is not equal. Some sources are more balanced than others. North daylight (in the northern hemisphere; south daylight in the southern hemisphere) is generally well balanced. But incandescent (tungsten) lights contain little in the blue end of the spectrum, and thus look slightly yellow.
Certain gems transmit strongly in the blue and red portions of the spectrum, but absorb the yellow. In daylight, such gems appear greenish, since our eyes are most sensitive to green light. But the same gem under an incandescent source is missing blue light. This tips the balance to the red side and the gem appears reddish. Such gems are said to display a change of color.
This effect was first noticed when a special type of chrysoberyl was unearthed in Russia’s emerald mines. According to the legend, the finder took it to be a fine emerald and decided to present it to the local king. In the candlelight of the palace, as the box holding the gem was opened, inside lay a small purplish gem resembling nothing so much as a small amethyst. Taking it to be an insult, the ruler ordered the man to be executed. But as the sun rose the following day, the king again looked at the gem and found it to be green and thus spared the miner’s life. Oh those capricious kings.
Apart from the standard factors used to assess gemstones, the quality and value of a color-change gem is judged by the strength of the color change. The ideals are an emerald green in daylight and a ruby red in incandescent light, but these are really never found. Most color-change gems go from a bluish green in daylight to a purplish red in incandescent light.
Double refraction & pleochroism
Gems that crystallize in any crystal system other than isometric (cubic) possess the property of double refraction, where a single beam of light is split into two component rays. The degree of separation of these two rays is termed birefringence. This optical doubling effect produces a twin image of facets when one looks through the gem. If the birefringence is strong enough, doubling may even be seen with the naked eye, calcite being the classic example.
A related effect to double refraction is an effect known as pleochroism. Not only can each ray be bent a different amount, but they may each be absorbed to a different degree, meaning that one ray takes on a different color than the other. This is pleochroism (pleo = many; chrome = color).
Pleochroism is entirely independent of birefringence. Dimetric (tetragonal, hexagonal) crystals have the potential to show two different colors, while trimetric (orthorhombic, monoclinic, triclinic) crystals can show up to three colors (but only two in any one direction).
Both pleochroism and double refraction result from distortions of the crystal lattice. Within isometric crystals (e.g., diamond, spinel, garnet), the structure is effectively a perfect sphere, where one direction is identical to any other. Thus light is affected the same in all directions. These crystals have only one refractive index and no pleochroism.
But if one takes that sphere and stretches it a bit, suddenly there are two unique directions. Looking down the end, it is still perfectly round, but perpendicular to the end, it is egg-shaped. These dimetric crystals will have two refractive indices and may show two colors (dichroism).
Now take that egg shape and distort it further by squashing it on one side. Suddenly you’ve created a shape where all three dimensions are different. These trimetric crystals have three refractive indices and the potential to display three different colors (trichroism).
As one rotates a pleochroic gem, the eye blends the two colors together, but differences will be seen in different directions.
When cutting strongly pleochroic gems (e.g., kunzite), lapidaries try to minimize the pleochroism and maximize the single best color. An exception is andalusite, where cutters try to orient the gem to get a pleasing mix of oranges, yellows and greens.
Many gemstones are pleochroic, but the two component colors seen by the eyes are so similar that the pleochroism is not particularly visible. Examples of weak to medium pleochroic gems are ruby, sapphire, emerald and chrysoberyl.
Due to their crystal structure, some gemstones do not possess pleochroism. This lack of pleochroism is extremely useful for species determination. For example, ruby and red spinel share many similar characteristics and one way of distinguishing between the two are by pleochroic tests. Notable examples of non-pleochroic gems are spinel, garnet and diamond.
This is the rainbow-like color effect seen in some gems and is caused by cracks or structural layers breaking up light into spectral colors. Fire agate is a gemstone that shows this phenomenon to good effect. When iridescence occurs in metallic hues (called schiller) in labradorite, it is commonly called labradorescence. In pearls, the subtle iridescence present is called orient.
Play of color
These are flashes of rainbow colors in opal that change with the angle of observation. This should not be confused with opalescence, which is the milky blue or pearly appearance of opal caused by the reflection of light.
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|Gemstone's Refractive Index|
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