Gemstones are rare minerals or rocks that are loved for their beautiful appearance and value. They are used for jewelry making, collecting, and decorative purposes. Gemstones are typically valued for their color, luster, hardness, and transparency. Semi-precious stones are a category of minerals that fall between rocks and gemstones in terms of hardness and luster. 

Opal is a type of amorphous semi-precious stone composed of silica spheres (SiO2·nH2O) and water. It has a Mohs hardness of 5-6. Opals are highly popular gemstones and are commonly found in Australia, Europe, and Japan. Opals come in a wide range of colors, including transparent, white, black, and iridescent variations. Among the various types of opals, the most sought-after are those that exhibit play of color. Only opals with play of color can be classified as gem-quality gemstones. A high-quality opal with play of color can encompass all colors found in gemstones, including red, golden yellow, blue, green, and purple. These colors also vary in intensity and depth within the gemstone. When viewed from different angles, opals with play of color exhibit a mesmerizing play of iridescent hues, creating a vibrant and captivating display. The supply of opals with play of color is limited, and black opals are particularly expensive. 

Depending on the intensity and vividness of their play of color, black opals can be valued at millions of dollars. Apart from opals with play of color, there are also common opals that exhibit a single color and lack play of color. 

Opals are formed by the arrangement of silica spheres, and the gaps between these spheres (Image 1) allow light to pass through. The diffraction of light through these gaps creates the phenomenon known as "play of color" or "fire." The effect of play of color varies with the observation angle and the light source, adding an element of ever-changing beauty to opals. 

Opals are found in various locations worldwide, including Australia, Ethiopia, Mexico, and Brazil. Australia is the primary source of opals, with over 95% of opals on the market originating from Australia. Australia produces a wide variety of opals, including crystal opal, milk opal, boulder opal, black opal, fossilized opal, and pink opal, among others. Mexico is known for its unique fire opals. Click on Opals and Where to Find Them to learn more about opals and their origins. 

Opals are formed by the condensation of silica and water, so they contain a certain amount of water. Normal opals typically contain around 5%-10% water, and opals within this moisture range have relatively stable structures. However, opals from Ethiopia have a higher water content, often exceeding 20%. The loose structure of Ethiopian opals, with numerous pores resembling a sponge-like surface, allows them to absorb water. This causes Ethiopian opals, which originally possess play of color, to become transparent or white, losing their fire. Sometimes, if left to dry naturally, the play of color may reappear, but this is not guaranteed. 

Opals come in a wide variety of types and appearances, exhibiting stark differences in their external features. If you are interested in learning more about different types of opals, click on Types of Opal to explore further.
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