Why does the garnet stone change color?

When you mention garnet, you usually think of a red mineral, but in fact, garnet has many varieties that differ from each other in color, composition and other properties. Today we will talk about one unusual and rare variety – a garnet with a color changing effect. Despite the recent history of the discovery of most deposits of these stones, they are becoming very popular in the jewelry market. This is due to the vivid manifestation of the color changing effect; in addition, stones with a high degree of transparency are often found. Garnets with a color-change effect are incorrectly considered to be an analogue of alexandrite – this stone is beautiful in its own way and even rarer. Also, garnets with an alexandrite effect have not been synthesized previously and are not refined (their color cannot be improved by this method), so when purchasing such a stone you can be sure of its completely natural origin. There are several varieties of minerals that are characterized by a color changing effect. These include alexandrite, sapphire, spinel, tourmaline, sultanite (a valuable variety of diaspora). Alexandrite, a color-change variety of chrysoberyl, is one of the first stones to exhibit this optical effect. In many ways, alexandrite is considered the standard of comparison for similar stones, despite the fact that the “classical” effect of changing color under different lighting is predominantly found in the Ural alexandrite from the Malyshevsky deposit. Surprisingly, in rare cases, the color change effect can be manifested in garnets of the pyrope-spessartine series. This is unusual in that the garnet group minerals are optically isotropic. Various optical effects are less characteristic of cubic crystals, which include minerals of the garnet group. This is due to the greater ordering of the atoms in the structure of such a crystal. Rice. 1. Garnets with alexandrite effect. It is worth understanding the difference between color-change effect and pleochroism in minerals. Pleochroism is the property of a crystal with anisotropy of optical properties to have different colors when viewed in different crystallographic directions. The color of a mineral is directly related to the interaction of electromagnetic radiation of various ranges with the external electrons of the atoms of the crystal lattice. It is most correct to associate color not with a specific impurity chromophore ion, but with the type of absorption. For transparent minerals, the absorbed part of the radiation determines the transmittance ranges, with which color is directly related. The phenomenon of pleochroism is associated with the uneven distribution of chromophore centers in a crystal along different crystallographic directions. Dichroism is characteristic of optically uniaxial crystals (corundum, tourmaline), and trichroism is characteristic of biaxial crystals (for example, kyanite). This effect does not occur in cubic crystals. Consequently, pleochroism is not found in minerals such as spinel, diamond, garnet, fluorite and many others. Accordingly, pleochroism is not observed in the garnet samples we studied. At the same time, the alexandrite effect is associated with the presence of chromophore impurities of chromium and vanadium and can be manifested in optically isotropic and anisotropic crystals. The color changing effect is associated with two transmission bands – in the blue-green (460-510 nm) and red regions of the visible spectrum (650-700 nm). In natural daylight, the proportion of blue-green radiation is large, and in the transmission spectrum the intensity of radiation of these wavelengths is also high, therefore, chrysoberyl with a color changing effect will have a corresponding color. In the spectrum of some sources of artificial lighting (for example, an incandescent lamp), the proportion of blue-green radiation is small, and the color of alexandrite will be due to its transmittance in the red region of the spectrum. Currently, an incandescent lamp is used as a source of artificial lighting, and a standard D65 gemological lamp for daylight. At the Gemological Center of Moscow State University, a batch of garnets with alexandrite effect from various known deposits was studied – Tanzania, Kenya and Madagascar. The size of the samples varied from 0.36 to 2.09 carats (see examples in the photo). The deposits of these countries are alluvial, and in them garnet with a color-changing effect is found together with other minerals. In Madagascar, garnets with a color-change effect are found in the Bekily province in the south of the island and adjacent territories. Interestingly, similar stones with different reverse colors were discovered there – yellowish-green and bright pink, as well as the recently popular Malaya garnet. In Kenya, the Taiti region is famous, where garnets with alexandrite effect have been known since 2009. The region also produces a wide range of precious stones, including tsavorite and ruby, which are in high demand in the market. Tanzanian color-shifting garnets were found in the Umba province, which produces commercial-grade rubies and sapphires, chromium-bearing green tourmaline, rhodolite and the rare garnet variety, umbalite. In addition to the listed sources of garnet with a color-change effect, similar stones were discovered in alluvial deposits of Sri Lanka, Afghanistan and Idaho, USA. Madagascar color-shifting garnets are considered the most valuable due to their almost blue color in daylight. For a long time, there was an opinion that garnets of such shades are not found in nature. The first mention of them dates back to 1998. The largest such garnet was sold for $1,5 million per carat. The most valuable quality in Madagascar stones is considered to be a strongly pronounced reverse color and a greenish-blue color, unusual for garnet, in daylight. In terms of reverse strength, Tanzanian stones are very close to Madagascar garnets. To determine the degree of manifestation of the alexandrite effect, there are special standards in accordance with TU 7026-001-26420171-94 “Natural alexandrites in raw materials”. They are used for alexandrite from the Malyshevskoye deposit and operate in Russia. According to the degree of change in the force of reversal, alexandrites can be divided into three groups:
1a – alexandrites with a strong alexandrite effect;
2a – alexandrites with a medium alexandrite effect;
For – alexandrites with a weak alexandrite effect. Due to the lack of standards for other stones with a color change effect, this classification may be applicable to them, including alexandrite effect garnets. To investigate the dependence of the intensity of the alexandrite effect in garnet on the concentration of chromium and vanadium, studies were carried out using optical absorption spectroscopy, microprobe analysis and visual observation in a standard Gretag Macbeth Judge II viewing room in standard daylight D65 (6500 K) and warm lighting (A – 2856 K), corresponding to an incandescent lamp. The observed colors were reflected in Munsell color coordinates. Rice. 2. Garnets from various deposits under different lighting with shades indicated in Munsell color coordinates. Rice. 3. Madagascar garnets under different lighting with shades indicated in Munsell color coordinates. Based on the results of microprobe analysis, the concentrations of chromophoric impurities – vanadium and chromium, which are associated with the phenomenon of the alexandrite effect – were identified. It turned out that garnets from Tanzania are characterized by an inverse correlation between vanadium content and transmission intensity in the windows 460-510 nm and 650-700 nm. All studied stones belong to pyrope-spessartine. The highest V content was recorded in a garnet weighing 1,20 carats (2,22 – 2,38 wt.% V₂O₃), for the remaining two samples with high transmission in these two optical ranges of 1,18 carats and 1,43 carats, the vanadium content is more than 2 times less (0,83 – 0,95 wt.% V₂O₃ and 0,79 – 0,90 wt.% V₂O₃ respectively). The highest intensity of transmission in the ranges was detected in two samples: Madagascar 0,36 carats and Kenya 2,09 carats. The Madagascar sample contained V₂O₃ varies in the range of 1,99 – 2,12 wt.%, in Kenyan – 0,56 – 0,69 wt.%. At the same time, there is no clear dependence of the vanadium content on the degree of transmittance in the two optical ranges. It can be noted that the “classic colors” of the alexandrite effect are found in garnets with the highest content of vanadium oxide. Rice. 4. Optical absorption spectra of the studied garnets with the effect of color change from various deposits. Thus, the concentration of vanadium in garnet with an alexandrite effect influences the degree of manifestation of the reverse. The distribution of impurities in the structure of natural untreated crystals is most often chaotic, so a direct correlation cannot be observed. Color change garnets are becoming increasingly well known on the market due to their distinct and largely unique color characteristics. Taking into account the fact that the color of such garnets is not affected by refining (in comparison with demantoid, for example), the value of such material is undeniable and will only increase. There are two opinions about the name of the gemstone “garnet”: some researchers suggest that it comes from the Latin term for “grain” and refers to the resemblance to the round shape of rough gemstones. However, at the microscopic level, garnet crystals are cubic in shape. Another opinion suggests that the name is related to the red fruit of the pomegranate tree. However, the natural color of the stone is not only red. Stones with similar structure and chemical composition vary significantly in color.

History of the pomegranate

He participates in numerous ancient tales and myths. In Greek mythology, this is, first of all, the myth of the abduction of Persephone by Hades. This gemstone is also mentioned in Jewish and Muslim traditions due to its special glow. Even today, pomegranates are considered a symbol of light, as well as loyalty and truth. Red garnets are mentioned in ancient legends, and because of their beauty, they have also long been used for jewelry. The Vikings decorated their tombstones with them to light the way for the dead to Valhalla. The crusaders decorated their armor with garnets, feeling safer under the protection of the stone. In the 18th and 19th centuries, garnets became a real fashionable decoration, with Bohemian garnets especially prized.

Characteristics of the stone

Despite the similar chemical composition and crystal structure, individual varieties of garnet have different colors. It depends on the metals they contain – manganese, calcium, and so on. As a result, the color of the stone varies from purple, chocolate, green, orange, yellow, pink to red. In total, about 38 different varieties of pomegranate are known. The garnet from Mozambique is considered a classic. Its color – the popular garnet red – at first glance resembles a dark ruby. Malayan pomegranate is still only produced in East Africa, where it was once discovered. African stone displays an intense orange, sometimes mixed with pink. Dark red pyrope is mined in South Africa and Yakutia. The color of rhodolite, which jewelers source from Tanzania, Sri Lanka and India, is also a beautiful red. Spessartine has a reddish or red-orange glow. It is mined in Australia, Brazil, Kenya, Madagascar, Mozambique, Pakistan, Sri Lanka, Tanzania and Zambia. Almandine red, brown, purple from Sri Lanka. Mongolia and Finland are the most popular. Hessonite shines with an intense honey-golden color. This beauty was popular among the Indians, Greeks and Romans. Hessonite is traditionally mined in Sri Lanka. Beautiful specimens are also obtained in Tanzania. Garnet can also shine in various shades of green if it is colored with chrome, a demantoid first found in Russia in the Urals.

Valuable properties of the stone

Garnet is the patron saint of passionate love, nobility and friendship. Considered a stone of wisdom, it is more intended for people who are pure and open; it does not favor dishonesty. Pomegranate is a talisman for travel and family life. The stone cleanses the aura of the premises well – and the head of its owner from bad thoughts.

How to care for a stone

All varieties of pomegranate should not be exposed to extreme temperatures, as temperature shock may damage them. The sun’s rays can also damage the stone. Jewelry should be cleaned in running water using a soft brush. Steam and ultrasonic cleaning are not recommended.