Geological classification

How to distinguish black opal from a fake?

Let’s start, as usual, with a definition. Opal is amorphous silica with the chemical formula SiO2*nH2O. One of the characteristic physical properties of this mineral is luminescence, the color of which depends on the color of the opal – it is white, blue and greenish in white samples, from green to brown in fire opals, and black opals play with all the colors of the rainbow under ultraviolet light.
Depending on their properties, types of opals are distinguished. Noble opal is classified as a precious stone and is characterized by a play of colors, as well as a high degree of transparency. Ordinary opal, on the contrary, is not characterized by the presence of opalescence and is often opaque. Based on color, fire and black opals are distinguished; they are the most highly valued in the jewelry market. In addition to the above, there is also boulderopal, kahalong, hydrofan, hyalite and other varieties of opal, but our article is not about that.
How can we understand what we are buying – a natural stone or a parody of it?
First, pay attention to the distribution of colors in the stone; if it is regular, then it is better to refuse such a purchase, but if the opal pattern is chaotic and does not repeat anywhere, then you can continue to study it. Twist the stone in your hands, if you can look at the opal from the side, then in synthetic opal you will see places where the layers are glued together, which look like even thin stripes. The zoning of colors in a stone can also indicate the origin of the opal – the clearer the color transitions, the higher the likelihood that we are dealing with a fake. The transparency of the stone is also not unimportant. Take a closer look to see if the dark substrate in the opal you are buying is visible. Not many people know that even our language can help in identifying a craft. How? It’s very simple – natural opal will not stick to the tongue. And of course, pay attention to the price of opal. Too low a price should raise suspicions of a fake. All these diagnostic methods are of course indirect. Only a qualified gemologist can give a 100% answer to the question of the origin of opal. Other publications In gemological practice, there are very interesting cases involving the diagnosis of jewelry inserts. But in addition to the rarity of color and the high cost of such stones, many pink stones stand out for one remarkable feature – they exhibit pleochroism, that is, depending on the position of the stone, it can have additional shades – orange or purple. Currently, jewelry stones are produced by two fundamentally technologically different methods – the High Pressure-High Temperature method (“HPHT”, High-pressure&High-temperature) and the Chemical Vapor Deposition method (“CVD”, Chemical vapor deposition). The HPHT method is the most tested classical synthesis method, which can use both carbon deposition on diamond from flux melts and catalytic reactions. In “CVD” synthesis, diamond growth occurs on a seed during the deposition of carbon primarily from a gaseous environment at relatively low temperatures and pressures. Jewelry and precious stones are just such a category of goods, when purchasing which you need to pay attention to many criteria. Sogdianite is a fairly rare mineral and can often be found as a collection material (in systematic collections), and it is extremely rare in jewelry. In this article I want to debunk a couple of myths about opals that literally infect society, and at the same time teach how to distinguish real opals from other similar minerals and stones. At the same time, I will try not to speculate on pictures of iridescent noble (not to be confused with fire) opals, which is what those who write on this topic usually do. So, myth one. For some reason, everyone thinks that opal must play with colors and shimmer like a mosaic in a kaleidoscope. Actually this is not true. Opal is hardly the only mineral in nature that does not have a crystal lattice. Opal consists of “microballs” (globules) of silica that bind a small amount of water (4-12%). So, in nature there are 130 (one hundred and thirty) types of opal, of which no more than 5-6 “shimmer” with colors. The rest have a single color, can be a dirty gray color and are completely opaque. Therefore, when pronouncing the word “opal”, it is necessary to clarify what we are talking about. Opalized wood, Ukraine Clarifications are not required only by search engines, when you enter the words “opal” hundreds of pictures pop up only noble disgraced! It is clear why opal is associated with the play of color. Well, let’s start with him then. There is a lot of information on the Internet about the nature of opal play. We will only dwell on the types of precious opals and how to distinguish them from fakes. At its core, opal is a fossilized silica gel. At the formation stage, it can permeate porous sedimentary rocks (sandstones) – this is how Australian Andamooka opals and Honduran “black” opals – tar sandstones – are formed. Honduran opalized tar sandstone. Or it can fill cracks in ferruginous rocks – Australian boulder opals. The color scheme of the game can be represented by 1, 2 or 3 colors (harlequin opal). In this case, the size of the colored spots can vary from 1 mm to the entire plane of the stone. The body of a solid opal (matrix) can be white or chocolate (Ethiopian opals) or black (Australian opals from the Lightning Ridge deposit). Chocolate opal, Ethiopia The most expensive is Full Spectrum Black Opal, Lightning Ridge, Australia. Myth 2: The main confusion arises when it comes to black opals. This is where I ask no one to worry – black opals are not imported into our country for 2 reasons. Extremely low production volume (Lightning Ridge only – listing this location is a guarantee of “real” black opal) and price. The price is not just high, but cosmic, unaffordable even for our home-grown oligarchs. If light Ethiopian opal costs $50/carat on the market (7 mm stone), then black opal, depending on the blackness of the matrix and the number of colors, already costs $1-5 thousand/carat. That is, a stone of the same size will cost not 2-3 thousand rubles, but 60-300 thousand. “What is it then that the pages of online stores are full of?!” – the observant reader will exclaim. Very simple. There is a technology for the so-called “smoking” of ordinary cheap light opals (carbon saturation). What gives “smoked” opals away is again the price – they are sold at the same price as light ones. They are very conditionally black. Pricing of precious opals is a rather complicated topic. You often see carefully sorted piles of rough stones from Ethiopian traders. The price starts from 2 euros/gram (a couple of years ago you could buy it for 0,5-1 euros) for small, cloudy, cracked pieces unsuitable for jewelry, up to 20 euros per gram – in fist-sized nodules. Today there are a lot of cheap imitations of precious opal. By appearance, without skills or special optics, noble opals can be distinguished from fakes by turning the stone in a circle and observing the play of color spots. Firstly, they should not have clear boundaries (the most primitive fakes), and secondly, when turning, these spots seem to go out and others “light up”, smoothly turning into each other, changing colors. If the insert (cabochon) or bead consists entirely of opal, then it is definitely Ethiopian (Ethiopia today has occupied 90% of the opal market in terms of sales, but not in prices, and has greatly reduced the price for them). Australian opals typically consist of a thin layer of opal on a host rock. Doubling and tripling of opals (mostly Australian) is widely used – the thinnest layer of opal is glued onto a dark layer of foreign stone (triplet – a transparent colorless plate or rock crystal, but more often synthetic, is glued on top, which makes such an opal eternal and reliably protected). But, as you understand, the opal in such an insert is not even half and these are not measured in carats. I even saw mosaic triplets – when the middle layer consists of a mosaic of 1-2 mm thin plates of noble opal. Depending on the skill of selecting such a mosaic, a cabochon measuring 10 x 7 mm can cost 3-10 dollars. Well, this is a wild abbreviation for noble opals. And now the “non-irridescent” opals. In general, the term “opalescence” refers specifically to single-color translucent opals and means a strong dispersion of light (a kind of haze, cloud). Along the way, let me teach you how to distinguish opals. The main thing is to find the chip. For beads, you need to look through a magnifying glass at the mouth of the bead; for fragments, everything is visible at once; for cabochons, you also need to look at the girdle with a magnifying glass. Opals have a unique “oily” sheen when chipped. Like melted butter. The more water in the composition, the more oily the shine, like after a good polish. Low water opal chips look like frozen oil. So, the next opals are fire opals. These are absolutely single-color translucent to transparent opals from yellow to brown-orange colors. Classic – carrot-orange. They don’t have any game. Variances, by the way, too. Such opals are supplied to the market by Mexico (more saturated and expensive) and Brazil (lighter and cheaper). Excellent fire opals were once mined at the Voznesensk deposit in Kazakhstan. But, times have passed and now they don’t dig there, but amateurs dig. Fire opal, Mexico Blue and white opals are generally inexpensive, but not very common. Instead, synthetics and imitations are used in costume jewelry. The exceptions are their varieties – andopal (greenish-blue opal from Peru) and hyalite – colorless opal. Hyalite is a mineralogical material, but the original form of its secretions in nature makes it interesting for designer jewelry. Landscape andopal, Peru Colorless opal – hyalite It is impossible not to mention the luxurious Kazakh dendro opals, of which there were quite a lot on the market 20 years ago. The prefix “dendro” means that at the same time as the opal, branches (dendrites) of manganese oxides crystallized in it. This dendropal has a clearly visible oily chip (fracture). Green opals, like chalcedony, are divided into 2 groups, depending on the chromophore – nickel (chrysopal) or chlorine – chloropal (ungvarite, kiwi opal). Chrysopal often coexists with chrysoprase in one block, as for example at the Pstan deposit in Kazakhstan. Chloropals have their names depending on the area of ​​production. Let’s finish with the famous pink opal from Peru. By the way, among all opals it is probably the most relatively inexpensive. Freshly mined chunks of pink opal just brought to the surface near a mine in Peru. Well, we very, very briefly went over the 12 main types of opals. Thank you all for your attention.

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