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Minerals and gems frequently present uneven color distribution that can be caused by different phenomena, such as:
Physical and chemical changes in the environment during crystal growth can cause variations of color of different parts of the crystal. The most common case is a variation of chromophore elements concentration that will provoke more or less intense color of different growth zones of the same crystal. Similarly, massive entrapment of solid or fluid inclusions can cause color variation on certain stages of growth in the host crystal, also resulting in formation of growth zoning.
Multi-colored tourmaline form Brazil. Field of view mm. Length 8,3 mm.
Colorless transparent beryl with sharp green emerald zone and two translucent milky zones due to fluid inclusions. Malysheva emerald deposit, Ural Mountains, Russia. Field of view 15 mm.
Chlorite phantoms in quartz from Brazil. Field of view 53 mm.
Color zones can be wide or narrow, easily seen by the naked eye or visible only under microscope, with well defined borders or gradual change of color. They can be equally developed in all directions or have very different thicknesses depending on crystallographic orientation, as a result of unequal growth velocity on different crystal faces.
Faceted gem sphalerites with color zoning. Two wide zones in the stone on the left and multiple sharp zones on the right. Field of view 20 mm each image.
Color zoning in emerald cut in perpendicular to C axis of the crystal. Field of view 5 mm.
Three well marked growth zones are present in this sawed beryl crystal from Urals, Russia. The growth starts in the central part, corresponding to emerald green color core with many phlogopite inclusions. It is followed by a large colorless zone, developing on one of the pinacoids and on prism faces. Finally one more green zone is formed, but only on one of the pinacoids. The border between colorless zone and the second green zone is also marked by phlogopite particles. Crystal lengh 24 mm.
In some gems, color zoning correspond to different saturation of the same color, product of quantity variation of the same chromophore element. In others, two or more totally different colors can be clearly visible, as a result of changes of chromophore elements during crystal growth.
Multi-colored “rainbow” fluorite from Argentina. Field of view 28 mm.
For most gems, uneven color distribution is a negative quality factor and cutters try to avoid color zoning visible from the crown. However, for some gems such as tourmaline, color zoning can become very beneficial when two or more different colors are clearly visible, creating bi-, three- or multicolored gem varieties.
Color zoning is an easy way for distinguishing synthetic flame fusion sapphires. In addition to curved growth lines, they usually have marked curved color zoning reflecting uneven incorporation of chromophore elements during crystal growth. Such curved color zoning is very different from straight color zones observed in many natural sapphires.
Straight color zones forming 120º in natural sapphire and curved color zones in synthetic Verneuil sapphire. Field of view: 7 mm (left) and 6 mm (right).
Sectoral color distribution
Growth sector is a part of crystal formed by one crystal face. For crystals forming under constant conditions, growth sector can be described as a pyramid with the base corresponding to the crystal face (growing front) and the apex in the center of crystal, corresponding to initial stage of crystal formation. However, in most cases, the shapes of growth sectors are more complex, due to changes of crystal habit and relative growth velocity of different crystal faces.
Schematic representation of growth zones (green) in comparison to growth sectors (red and blue). Note that in cube only one type of growth sectors is present, while hexagonal prism is composed by growth sectors corresponding to pinacoid (red) and prism (blue) faces.
DiamondView image of clearly marked growth sectors in synthetic HPHT fancy intense yellow diamond. Yellow sectors correspond to octahedron faces, while more greenish narrow sectors correspond to cube faces. The difference in trace-element composition is revealed in this case not in visible light, but in color and intensity of fluorescence under short wave UV light. Photo IGE.
Different growing planes in crystal lattice have different capacities to incorporate impurities. As a result, growth sectors formed by different crystal forms usually have slightly different chemical composition, which will also cause variations of physical properties, such as color, refractive indexes, crystal structure parameters, concentration of defects and inclusions, etc. Moreover, tensions on the borders of adjacent growing sectors may cause dislocations, fissures, splitting or twinning of crystals (Krasnova, 1995).
Biterminated “hourglass amethyst” crystals from Morocco, rough and polished. Colorless growth sectors correspond to prism faces, while violet sectors are formed by rhombohedra faces. Note also the presence of syngenetic hematite red ribbon-like inclusions, concentrated exclusively in prism growth sectors – evidence that iron was incorporated in quartz in ion form only in rhomboedra sectors and formed separated mineral phase in prism sectors. Rough crystal length 25 mm, cabochon lengh 24 mm.
Sectorial color distribution is a common phenomenon in many gems. It is even responsible for a special gem variety such as ametrine, where amethyst and citrine coloration is observed in different growth sectors of the same quartz crystal. Curiously, synthetic ametrine has color zoning as a mechanism for formation of two different colors within the same crystal; thus observation of crystallographic orientation of the border between two colors can help for identification of this synthetic material.
Zonal and sectoral color distribution in sapphire. From lower to upper side, in direction of crystal growth, three zones can be seen – dark, clear and dark again. Note different intensity of blue color within the same zones, corresponding to growth sectors of different crystal faces. Field of view 5.6 mm.
Comparison of natural ametrine with sectoral color distribution (left) to synthetic ametrine with normal color zoning (right).
Irradiation, natural or artificial
Different types of ionizing radiation can cause creation of optically active centers in minerals changing their color. Radioactive decay of unstable isotopes in mineral deposits is the cause of naturally occurring green diamonds, blue topazes, smoky quartzes and many other minerals. Irradiation is also one of most common treatments for gems, applied to wide range of gem materials to produce or intensify their color.
Natural irradiation in many cases generates non homogeneous coloration because certain parts of the crystal may stay in closer vicinity to the source of irradiation – minerals with high concentration of radioactive elements. Such are the cases of black halos around uranium and thorium minerals included inside quartz crystals or green spots on some diamond crystals.
Quartz crystal with dark areas of smoky quartz, produced by natural irradiation around of inclusions of uraninite (irradiation halos). Crystal length 58 mm.
Penetration power of different types of radiation is very variable. Gamma rays and neutrons go easily through large crystals; they can be used in rough and cut stones and provide homogeneous color distribution. Beta particles only penetrate to a very small depth in minerals. When they are used for diamond treatment, produced color concentrates in a very thin layer close to surface, usually on pavilion. Finally, alpha particles have very low penetration power; they are stopped even by a sheet of paper, so they are not suitable for gem treatments.
Image: Color concentration on the pavilion surface of blue diamond irradiated by electrons.
Diffusion, natural or artificial
Diffusion is another process that can cause uneven color distribution in gems. In this case, atoms of certain chemical elements move through the crystal lattice of the host mineral, changing its color. Diffusion process occurs when there is a very significant difference in concentration of chemical elements between two different phases, it is highly favored by heating and, when naturally occurring, it usually has a very local scope.
As a treatment process, diffusion of titanium has been widely used to produce blue color in natural and synthetic sapphires for decades. This treatment produces a thin blue layer in colorless or pale colored stones close to their surface. Furthermore, starting in early 2000s, a new process involving deep diffusion of beryllium (and maybe other light elements) has been introduced for corundums. It is applied to a wide range of colors, with yellow, orange and padparadscha as most common resulting colors. Small atomic radius of beryllium makes possible its deep penetration inside preformed gemstones, up to several millimeters, in contrast to traditional titanium diffusions that usually reaches less than one millimeter depth.
Titanium diffusion halo around of rutile inclusion in strongly heat treated sapphire. Field of view 3 mm.