https://www.gemsociety.org/article/how-do-opals-form/
Chemically speaking, opal is a form of hydrated silica, with the chemical formula SiO₂ · n H₂O. Unlike most gemstones, opal isn't crystalline. That means the silica doesn't form in a nice, neat crystal structure. Instead, it forms as microscopic, amorphous spheres.

https://www.opalsdownunder.com.au/learn/how-is-opal-formed/
If the process allows spheres to reach uniform size, then precious opal commences to form. For precious opal the sphere size ranges from approximately 150 to 400 nanometres producing a play of colour by diffraction in the visible light range of 400 to 700 nanometres.

https://geologyscience.com/gemstone/opal/
Hydrophane opal (precious opal) dried out (Tertiary; Ethiopia) Opal from Yowah, Queensland, Australia. Definition: Opals are a type of mineraloid, which means they lack the crystalline structure of typical minerals.Instead, opals are composed of tiny spheres of silica, often arranged in a closely packed and orderly manner. What sets opals apart from other gemstones is their remarkable play of

https://www.gia.edu/opal-description
During dry periods, much of the water evaporated, leaving solid deposits of silica in the cracks and between the layers of underground sedimentary rock. The silica deposits formed opal. How Opal Forms. Opal is known for its unique display of flashing rainbow colors called play-of-color. There are two broad classes of opal: precious and common.

https://opalgalaxy.com/how-is-opal-formed/
There are two different categories of opals: Precious Opal and Potch or Common Opal. Precious Opal is the sort after colors and patterns that you can see in Opals. These make up about 15% of all opals discovered in Australia. The other 85% of Opal discovered is Potch or Common Opal. Opal cutters and polishers often discard this colorless stone

https://gem-a.com/types-opal-gemstone/
The banded variety of common opal which can form in opalised fossils or in veins. Dendritic or Moss Opal. These moss like (dendritic) inclusions are formed of different iron minerals encased by the opal producing these aesthetic designs. ... Basil Anderson sums up the identification of precious opal quite wonderfully with this quote, included

https://www.gemsociety.org/article/opal-types/
What is an Opal Type? Opal types are primarily based on the gem's body color. This is the color that shows under a precious opal's play of color, the flashing or moving colors across its surface.However, sometimes the form, matrix, or inclusions define the type. (Opals that show play of color are called "precious opals," while those that don't are called "common opals").

https://en.wikipedia.org/wiki/Opal
Opal is a hydrated amorphous form of silica (SiO 2 · n H 2 O); its water content may range from 3% to 21% by weight, but is usually between 6% and 10%. Due to its amorphous property, it is classified as a mineraloid, unlike crystalline forms of silica, which are considered minerals. It is deposited at a relatively low temperature and may occur

https://www.minerals.net/gemstone/opal_gemstone.aspx
Black Opal - Precious Opal with a black, dark blue, dark green, dark gray or similar darkly colored background or base color. Black Opal is the most valuable form of Opal. Boulder Opal - Precious Opal from Queensland, Australia, found in the cracks of, or as coatings on, ironstone or sandstone boulders.

https://www.opalsdownunder.com.au/learn/what-is-opal/
Opal, a crystalline gem mineral, takes form through the solidification of gelatinous or liquid silica within cracks and voids, often created by decaying organic matter, wood, crustaceans, and bones. Its highest value resides in its "black" forms, embodying a significant water content. Represented by SiO2 plus H2O, opals can encompass up to

https://www.opalauctions.com/learn/did-you-know/difference-between-precious-and-common-opal
When opal forms, whether common or precious, other chemicals can get caught in the mix. When these elements get stuck inside opal's silica spheres, they give the stone a new background color or body tone. Precious opals can have a few different body tones, including white, black, yellow, and blue, to name a few.

https://www.opalauctions.com/learn/opal-information/types-of-opals
In precious opals, body tone is the relative darkness or lightness of the stone's background. Body tone in common opals is simply based on the stone's overall color. ... These opals can form in fossils of animal skulls, shells, pineapples, and even 100-million-year-old squid called opalized belemnite. While mostly found in Australia, miners

https://www.gemsociety.org/article/opal-jewelry-and-gemstone-information/
Matrix opal consists of specks of precious opal in a rock matrix, usually sandstone. This type of opal is often dyed black to enhance the color play. Ironstone opal forms in a hard, brown, compact type of sandstone. Matrix opal may also consist of layers or stringers of opal in a rock matrix. Also called mass opal. Other Terms

https://www.gia.edu/opal-quality-factor
Opal Quality Factors. This large Australian light opal cabochon weighs 117.85 carats. Small play-of-color flashes of orange and green predominate in a pinfire pattern, with spots of a more muted blue over a grayish bodycolor. Individual opals can vary widely in appearance and quality. As diverse as snowflakes or fingerprints, each gem can

https://rockchasing.com/types-of-opal/
The opal forms in thin veins or patches within the rock, leading to a variety of shapes and sizes, often resulting in highly unique and natural pieces. ... Crystal opal is a type of precious opal known for its transparency and the brilliant play-of-color within. What makes crystal opal distinct is its see-through body, which allows light to

https://www.philophrosyne.com/blogs/jewelry-guide/types-of-opal
The most common types are precious opal, common opal, fire opal, boulder opal, matrix opal, and crystal opal. You can identify them by looking at the color, pattern, and play of color of the stone. ... Opalized wood is a type of opal that forms when opal replaces the organic material in wood. Since it's very fragile, be very careful when

https://jewelrytalk.com/gemstones/opals/
In a boulder opal the host rock forms within the seams and fractures of the opal yet with matrix opal, the precious opal is intimately mixed with its parent rock. ... Ethiopian Opal - Precious opal, fire opal, and beautiful common opals are often found in Ethiopia. Ethiopia took its place in the market in 1994 and has since taken a considerable

https://rockseeker.com/collecting-opals/
Opal is deposited at low temperatures and pressures compared to crystalline forms of silica. The moisture percentage ranges from 3% to 21%, but most are on the lower side. 6%-10% is most common. ... Precious opal is an amazing stone, and it usually has a cost to match. The difference is that this form of opal has a play of color that moves

https://rockandmineralplanet.com/all-about-opal-uses-properties-color-and-worth/
Precious opal is a more precious and rare form of opal. It has what is called play-of-color, or iridescence. Precious opal is a phenomenon of colors from the light spectrum reflecting on a silica surface. This phenomenon is similar to what you would see on a soap bubble floating through the air reflecting light.

https://australianopalcutters.com/blogs/news/beginners-guide-what-are-the-different-types-of-opals
These forms of opal can be found in countless lands across the world, although Australia is regarded as its 'home', as around 95% of the world's precious opal is found here. The Difference Between Precious and Non-Precious Opals. Precious opals are often more revered for the spectrum of colours that can be observed through the stone, whereas

https://www.gemstones.com/gemopedia/opal
Opals gained widespread popularity after it was discovered in 1869 in Australia. Opal has no crystal structure, and it is an amorphous form of hydrated silica. It comes in two forms, precious opal that displays the play-of-color phenomena, and common opal that does not.

https://www.opalauctions.com/learn/how-tos/opal-value
Crystal opal is the second most valuable opal after precious black opal. This precious opal type is colorless, transparent, not milky, and ranks at N7 to N8 on the body tone scale. Here are the crystal opal prices by carat weight ranges: 1 to 5 carats: $2 to $6,000 per carat. 5 to 10 carats: $2 to $6,400 per carat.

https://www.wikihow.com/Identify-Raw-Opal
This opal forms in volcanic rock or in ore deposits. Advertisement. ... In the case of precious opals specifically, these background colors are complemented with a special play-of-color feature. Known opal colors: Colorless, white, orange-red, opaque white, pink, green, blue, yellow, brown, and more 2. Opals can appear clear or opaque.

https://www.click2houston.com/news/texas/2024/06/17/ms-opal-lee-is-known-as-the-grandmother-of-juneteenth-heres-how-she-started-her-activist-journey/
Opal Lee's dream of seeing Juneteenth become a federal holiday was finally realized over the summer, but the energetic woman who spent years rallying people to join her push for the day