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How to distinguish synthetic spinel from natural?

During the Hong Kong Jewelry Exhibition in September 2008, many “Royal Burmese” spinel stones were sold. During post-sale inspection, several stones were suspected of being inauthentic and were sent for testing to SSEF, the Swiss gemological institute. As a result of tests, the stones were recognized as synthetic spinels. That same year, these spinels were offered (and are still actively sold at the Chanthaburi market) in Bangkok during the September jewelry show. The production of synthetic spinels is not new, and the problems of their identification are not new. Nassau describes in his book Gems Made by Man (1980) that the production of synthetic spinels actually began in the mid-18th century when spinel crystals accidentally grew while trying to grow rubies. Recently, Mühlmeister et al. (1993), Schaub (2004) and Notary and Grobon (2004) have well described the properties of such stones. Due to the fact that in recent years a certain amount of red spinel of very good quality has appeared on the modern market with a very attractive price for sellers, it is not surprising that synthetic spinel has again made itself known. Unlike synthetic spinels made using the Verneuil method (they are mostly light blue, green-yellow and colorless), the newly presented samples turned out to be very difficult to distinguish from natural ones. The external resemblance of synthetic spinel to natural spinel of the best quality turned out to be simply amazing. Only careful examination under a microscope and chemical analysis can show the evidence of its synthetic origin. Traditional gemological methods, such as determining the refractive index and absorption spectrum of these flux-grown spinels, are not helpful. The characteristics are absolutely similar to the corresponding characteristics of natural stones. When studying images in a polariscope, these synthetic spinels demonstrate the absence of birefringence, as befits natural spinel, although when using cross filters they show various anomalous transitions characteristic of zones with increased voltage without reference to inclusions. A similar effect can be observed in natural spinels, but only around inclusions. Under ultraviolet LW and HF, synthetic stones show orange-red fluorescence, which is characteristic of natural samples, but along the edges of the ribs a very light milky or yellowish-orange fluorescence can be observed, which is completely unusual for natural spinel. One way or another, all these methods do not seem to be reliable for confidently identifying flux synthetic spinel. Research data under a microscope: All synthetic spinel samples were of exceptional purity. However, all samples contained microscopic tubular cavities containing jagged foreign bodies of a very dark orange or brown color, which were flux residues. All samples showed the presence of gas bubble inclusions formed due to inhomogeneity of flows during cooling. In addition, one of the samples showed, in addition to inclusions of flux residues, the presence of a hexagonal metal inclusion, which owes its origin to the platinum crucible in which the spinel grew. Natural red spinels, especially from Burma (Myanmar), do not differ in purity, often the inclusions in them are very significant, and show healed cracks with a large number of microscopic octahedral crystals with negative optical characteristics, and various crystalline carbonate inclusions with a rounded “alluvial” shape. Often natural Burmese spinel contains brownish iron hydroxide in large cracks and cavities, which should not be confused with the flux residues in synthetic spinels described above. Chemical analysis results: The chemical composition of the analyzed red spinel of synthetic origin is at first glance very similar to the composition of natural red spinel. Unlike Verneuil spinel, which shows a high concentration of aluminum, these synthetic spinels have a stoichiometric Mg:Al ratio and are similar to natural ones. This explains the complete coincidence of the refractive index and density indicators with the corresponding parameters of natural samples. In addition to these main components, differences in the concentrations of chromium (0,5 – 2,5 wt% Cr2O3), in combination with traces of iron, vanadium, nickel, zinc and gallium, were identified in synthetic stones. Platinum was detected in one sample due to the metal flakes mentioned. These elements (except platinum) may also be present in natural spinels. The main distinguishing feature is the low concentration of zinc (0,01 – 0,02 wt% ZnO). The concentration of zinc oxide in natural spinel is usually about 10 times higher (Schaub, 2004). The Raman spectrum shows a distinct broad peak (at 406 nm) for fluxed synthetic spinel, which is absent in the spectrum of natural samples.
Similar peaks are observed in Verneuil spinel. The presence of this peak is apparently explained by deformations in the cubic crystal structure of the stone. In addition, excitation of the samples with a green laser (514 nm) resulted in strong photoluminescence with chromium-related peaks. However, these emission peaks are much less structured than those of natural chromium-containing spinel (Notary and Grobon), which provides another good opportunity to find differences between synthetic and natural spinel. Uncut crystals of flux-grown synthetic spinel also show significant similarities to natural crystals. Like natural ones, they show the triangular shapes of natural edges. Even with careful examination, a specialist buyer of raw crystals is unlikely to be able to notice the differences between the most beautiful synthetic and natural crystals. Only careful examination using a powerful magnifying glass or microscope can help. But the most reliable way to identify whether spinel is natural or synthetic is to contact any serious gemological laboratory that has the necessary equipment for analysis. Natural gem-quality spinel has been rapidly gaining popularity in recent years as a rare precious and “semi-precious” stone. A situation arises in the market of increasing demand in conditions of limited supply. In the world mineralogical community it is known under the English term “spinel”. The name of this mineral has Latin roots and comes from the term “spinella”, which means “thorn”. This is a direct indication of the crystal shape, octahedron and dodecahedron, in the form of which the mineral is formed in nature. Its properties as a precious stone are remarkable. Spinel has a strong glass luster, which makes it easy to polish and very beautiful to cut. Another excellent property is the high dispersion, which is 0,026. The hardness of the mineral is very high, about 8 on the Mohs scale, which is close to the values ​​of beryl, topaz and slightly less than that of noble corundum, ruby ​​and sapphire. Among the characteristics of this stone, the most important is color; red shades, including pink, and blue are especially valued. In addition, purity, quality of cut, weight and origin influence the cost. We will tell you more about how to choose this noble stone in this buyer’s guide, and in the second part of the guide we will summarize how much cut natural spinel costs.

Colors and types of spinel

The correct answer to the question: “What color is spinel?” is “almost any.” Her palette is incredibly diverse: pink, red, orange, blue, blue, purple, black. Sometimes there are colorless, yellow and green stones. The color of the mineral depends on the content of impurities of various elements in the structure. The bright red variety is considered the most valuable. Rare neon blue cobalt spinel is also highly prized; when cut, it rarely exceeds a couple of carats. Next, we will look at the features of each color type of spinel:

Red

The most valuable natural variety, often referred to in literature as “lal”. Its properties are such that, according to its external description, red spinel is extremely similar to a scarlet ruby. It is sometimes referred to as ruby ​​or chrome spinel because the color comes from the presence of chromium in the structure. The red color can be of different saturations, from light to dark, almost black, sometimes the stone contains additional shades: orange, brown, purple. Being present in large quantities, they significantly reduce the cost of a sample, so pure red colors of medium and high saturation are most suitable for investment purposes. In the photo: a rich red specimen of natural red spinel Of particular interest lately has been the appearance on the market of single specimens of Jedi spinel; it seems to glow from within and is of Burmese origin. As a rule, the weight of specimens does not exceed 2 carats.

Pink

A worthy noble stone with a delicate and romantic pink hue, it is suitable for creating world-class women’s jewelry; surrounded by white diamonds, pink spinel is especially delightful. The most famous deposit is located in the Pamirs, Tajikistan. Pamir pink spinel is good due to its pure light pink hue, without additional tones. This is valuable because the color of such a spinel is close to the tone of a pink diamond: a little cool, not bright, but not pale either. Other deposits include Vietnam, Burma, Tanzania. Specimens from Tanzania (Mahenge) can reach a rich neon red-pink. An untrained eye will consider such a stone to be synthetic; it seems to glow from within. Avoid purple and orange shades, they will reduce the value of pink spinel.

Orange

Orange is a rather rare color for this mineral, and may have a red or yellow tint. When cut, it looks bright and juicy and goes well with the color of yellow gold. Orange and orange-red spinel are sometimes referred to by the commercial name “rubicell”. This look is not dark, sometimes it looks neon.

Blue

One of the most valuable varieties. If the coloration of the sample is caused by cobalt impurities, the commercial variety will be referred to as “cobalt” accordingly. Color – in shades of blue, from dark blue to bright neon. Sometimes it can be found under the term ganospinel, ganite. Often, blue spinel can have a grayish or purple tint, and even have a subtle color change effect depending on the type of lighting. Excessive density of blue is a price-reducing factor, so when choosing spinel we recommend avoiding dark samples.

Blue

This blue stone is a very attractive gemstone. It is not very common, a pure bright, sometimes neon sky color is incredibly good, extremely rare. It is usually associated with the Luk-Yen deposit in Vietnam. Due to its rarity, neon blue spinel of this origin is sought after by mineral and gem collectors.

Purple

The pleasant purple color of spinel, sometimes with a little pink or steel gray, looks elegant and delicate in jewelry. In light lilac and lavender specimens, the play of color on the edges – dispersion – is beautifully revealed.

Black and gray

Black spinel is an inexpensive, common semi-precious stone, generously used in silver jewelry, beads and costume jewelry. This variety is interesting because when cut it looks like a black diamond. It ranges from gray transparent to black opaque, however the highly polished black variety has an excellent shine. Pictured: steel spinels

Green

This green stone is little studied and rare. Its color varies from bright green to dark green, the color is often weakly saturated, slightly grayish. Being caused primarily by chlorine impurities, this species may be referred to as chlorospinel. Offers of truly attractive jewelry pieces are rare on the market.

White (colorless)

The rare white variety of this gem appears colorless, however, sometimes there may be a slight shade of pink, gray, or lavender, which can appear in different lighting. The structure of colorless spinel has no or minimal coloring impurities and is highly transparent. In the photo: colorless cushion-cut variety Like other colorless stones, for example, sapphires, beryls, topazes, danburites, when cut it resembles a diamond. However, due to different properties, lower dispersion and gloss values, it plays more modestly.

Cutting

Spinel has a strong glass luster, it takes polish well, and is very beautiful when cut. Also a big advantage is the high dispersion. It must be taken into account that in saturated red, pink and blue stones it practically does not appear and is revealed only in cut specimens of light, gray, light pink, bluish colors. The following cut shapes are popular: Much less often, individual specimens have the shape of a circle, marquise, trillion, or heart. The issue of selecting pairs and sets of three or more stones requires separate discussion. For more common types (gray, for example), with stones weighing up to 1,5-2 carats, it is relatively easy to make a selection. However, for rare red, pink, purple, blue spinels from 2 carats, selecting a good pair with the most similar characteristics is a real quest for a gemologist. Stones may vary in color, clarity, and cut, so to ensure a perfect match, the stones are recut. Despite the sometimes significant weight loss, the result is worth it, so pairs and sets of spinel have an additional markup of 10-20% and are more expensive.

Cleanliness

Only a small part of the mined raw materials in crystals and fragments is of jewelry quality and is suitable for cutting. The spinel gemstone must have high purity, a small amount of gas-liquid, mineral inclusions, small cracks that do not impair the appearance of the sample are acceptable. In the presence of large, noticeable inclusions and cracks, the value of spinel decreases.

How to distinguish real spinel from a fake

This is a pressing question, since there are a large number of imitations and fakes on the market today. Among artificial materials, these are primarily glass (the so-called jewelry pastes), glass ceramics, and cubic zirconia. Particular attention should be paid to the synthetic analogue – artificial aluminum-magnesium spinel, obtained in laboratory conditions. Thanks to the addition of coloring impurity elements during the synthesis process, scientists obtain a wide range of colors and shades: green, blue, red, pink, purple, colorless, etc. The most reliable way to distinguish a natural mineral from a counterfeit and synthetic analogue is in the laboratory, by its characteristic features using gemological methods and related equipment. All lots presented in the Gem Lovers catalog are carefully studied in advance by the company’s gemologists. The buyer can also order the production of a certificate or expert opinion from a reputable certification center. Among natural stones, minerals such as garnets (almandine, pyrope, rhodolite), tourmaline rubellite, zircon hyacinth, and fancy sapphires resemble natural red and pink spinel. Gems similar to blue and blue spinel: blue sapphire, Cambodian zircon, indigolite tourmaline. Go to part 2 of the buyer’s guide, in which we talk about the cost and give real prices per carat.

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