Opal is a unique mineraloid formed from a substance called silica gel. The best type of opal is called "precious opal," that shows play of color similar to a rainbow when you hold it against the light. Miners find these precious opals in different parts of the world, like Slovakia, Ethiopia, Australia, Brazil, Mexico, United States, and Java. Ethiopia became a large producer after finding tons of opal in Wegel Tena in 2008. Challenging Australia's dominance in the industry.
The play of color in precious opals happens because of light refracting in the voids of tiny, well-arranged silica spheres in opal-A and lepispheres in opal-CT. Usually, you see different colored patches side by side. Sometimes, the whole stone has a network of color that moves together, and these stones are natural "photonic" crystals. Unlike regular crystals that bounce X-rays, these photonic crystals bounce wavelengths in the visible range of the spectrum, creating that colorful show. The colors in precious opals can be arranged in different patterns, like specks (pinfire), flames, bands, and even patches that look like polygons (harlequin opal). Sometimes, you see straight black lines or bands crossing the patches, and that's because of certain arrangement of spheres in the structure.
Unlike precious opals, common opals usually don't have this colorful show because the silica spheres aren't neatly packed. Although some opals are colorless, most of them have a basic base color like white, black, gray, brown, yellow to orange (fire opal), blue, green, or violet. These colors come from tiny minerals inside the opal. For example, iron gives colors like yellow to brown (fire opal), copper gives a deep blue (Peruvian opal), and nickel adds a green tint (chrysopal). Other things that can add color are color centers (like purple fluorite inclusions) and organic compounds (like quinones in pink opals.
Opals discovered in Wegel Tena are like no other, boasting a unique mix of elements, with some even breaking records for high concentrations of Ba. Unlike opals you'd find elsewhere, these gems from Wegel Tena tend to have more chemical impurities. Their distinct geochemical signatures make them stand out from opals found all around the world, but here's the twist – they also come with quite a bit of variation.
Now, let's talk correlations between different elements like Al+Fe vs Na+Mg+Ca+K+Ba, Al vs Ca, or Ba vs Ca. It seems the opals' chemistry dance is influenced by the crystallochemistry of opal. Even though Wegel Tena opals aren't the most organized at the microscopic level, they're mainly made up of these cool three-dimensional Si-O bond networks.
Here's where it gets interesting – the concentrations of various elements in these opals follow particular trends. This hints at a mix-and-match fluid responsible for making these gems, pulling in at least two sources of silica: one from feldspar turning into clays and another from the weathering of volcanic glass. Microscopic examination reveals the presence of numerous plant fossils within some of the opals, alongside pedogenic features such as abundant clays, desiccation characteristics, and grain size sorting. The suggested hypothesis proposes that Wegel Tena opals formed during a hiatus in the emission of ignimbrite ashes, allowing for soil development to support plant life.
Ethiopian opals, which are primarily Opal-CT, are scientifically known as hydrophane due to their unique ability to absorb water. This property is attributed to their porous nature.
The formation of Ethiopian opals is linked to the country’s volcanic activity. The opal source is found in rocks created from cooled magma or lava. Water and minerals movement resulted in silica gel that became Ethiopian opals. When these opals are exposed to water, they can absorb large amounts of it, causing a change in color and transparency. The color will eventually return.
The porous nature of these opals is due to the way they form. Instead of being formed underground with silica-rich water like Australian opal, leaving opal in cracks and voids, Ethiopian hydrophane opals form in nodules in stratified igneous rocks due to volcanic activity. Rather than sedimentary rocks, volcanic ash has played a more critical role in creating the hydrophane Ethiopian opals.
The term “hydrophane” originates from Greek and translates to “water-loving”, which aptly describes Ethiopian opals’ typical water absorption. This unique characteristic adds to their mystique and collectability.
Opal-CT: is composed of extremely tiny cristobalite-tridymite aggregates, known as lepispheres. Volcanic activity is behind these hydrophane opals’ formation, and they come out knobby-formed (like lump-shaped balls) rather than seam-formed like most opals.
Opal-A, Opal-CT, Opal-C, Opal-AG, Opal-AN, α-Quartz, β-Quartz, Chibaite, Coesite, α-Cristobalite, β-Cristobalite, High-Quartz, and Low-Quartz are distinct types of silicates, each exhibiting unique characteristics due to their differing internal structures and conditions under which they form.
α-Quartz and Alpha-Quartz: These are the same phase and represent the stable form of quartz at low temperatures. It’s the most common polymorph of SiO2 and is found in many geological settings. It's the stable form of quartz at temperatures below 573°C (1063°F). It has a trigonal crystal structure and is the most common form of quartz found in nature, used in many industrial applications and as a gemstone.
β-Quartz and Beta-Quartz: Again, these are the same phase. β-Quartz is the form that quartz takes at high temperatures (about 573 °C at one atmosphere of pressure). The transition from α-quartz to β-quartz is abrupt and is accompanied by a linear expansion. It has a hexagonal crystal structure. Upon cooling below 573°C, β-Quartz transforms into α-Quartz.
Chibaite: This is a rare silicate mineral. It’s a silica clathrate, meaning the silica hosts or traps various hydrocarbon molecules, such as methane, ethane, propane, and isobutane.
Coesite: This is a high-pressure silica polymorph that is stable at pressures of 3–10 GPa and temperatures less than ∼3000 K. It’s often found in rocks that have experienced shock metamorphism. Formed at pressures greater than those typical at the Earth's surface, such as during meteorite impacts or deep within the Earth's mantle. It has a monoclinic crystal structure and is denser than quartz.
α-Cristobalite and β-Cristobalite: α-Cristobalite is usually metastably quenched as twinned crystals from the cubic β-cristobalite that is stable above 1,470°C. The transformation from α-cristobalite to β-cristobalite occurs over a temperature interval from around 1300 °C up to the melting point of SiO2.
High-Quartz: This is the more symmetrical form quartz takes at sufficiently high temperatures (about 573 °C at one atmosphere of pressure), but the relationship is pressure-sensitive.
Low-Quartz: This is the stable phase of quartz at low temperature and pressure, including normal Earth surface conditions.
Opal-A: Is amorphous, meaning it lacks a well-defined crystalline structure. It is the most common form of opal and typically exhibits a play of color due to its internal structure, consisting of silica spheres arranged in a random or sometimes orderly manner. The formation of Australian opal is influenced by diatoms, a form of algae. When diatoms die, they often form layers at the bottoms of lakes, bays, or oceans. Their cell walls are made up of hydrated silicon dioxide which gives them structural coloration and therefore the appearance of tiny opals when viewed under a microscope.
Opal-CT is composed of extremely tiny cristobalite-tridymite aggregates, known as lepispheres. This type of opal is primarily found in Ethiopia. Volcanic activity is behind these hydrophane opals’ formation, and they come out knobby-formed (like lump-shaped balls) rather than seam-formed like most opals The difference in internal structure compared to Opal-A, contributes to the unique optical properties of Opal-CT, including its own version of the play-of-color phenomenon.
Opal-C is a type of microcrystalline opal made of cristobalite, sometimes called lussatine. It represents a transitional state in the formation of diatomites and radiolarites from opaline skeletons and is found in nodular concretions in sediments, known as menilite. It is composed of tiny crystals of cristobalite. It represents a more crystalline form of opal compared to Opal-CT. This form occurs when Opal-CT undergoes further recrystallization.
Opal-AG refers to amorphous opals with a gel-like structure. A gem-quality opal with a well-defined internal structure that results in its characteristic play of colors. The "AG" stands for "amorphous gel," indicating its amorphous nature but with a structure that allows for gemstone quality. This is the structure of potch opal, precious opal, and fire opal.
Opal-AN stands for "amorphous non-gel" opal. Unlike Opal-AG, Opal-AN lacks the play of colors and is more common as a non-gem variety. It still maintains an amorphous structure. A common example is hyalite or glass opal.
From the volcanic landscapes of Ethiopia and Java to the ancient seabeds of Australia, Brazil, Mexico, Slovakia, and the United States, opals capture the essence of our dynamic planet. Each opal, whether it’s an Opal-A from Australia or an Opal-CT from Ethiopia, tells a unique story of geological transformation. The play-of-color, a spectacle of shifting hues dancing under the light, is a testament to nature’s artistry, painted on a canvas of silica spheres or lepispheres. These gemstones, formed over millions of years, serve as a vibrant reminder of our planet’s past and a symbol of its natural beauty. As we continue to explore and understand these precious stones, we gain not only a deeper appreciation for their aesthetic appeal but also a greater insight into the geological processes that shape our world.
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