What is Opal and What Class of Mineral Does it Belong To?

what is opal?

Have you ever seen an opal in person? It’s a stunning gemstone, often referred to as “the queen of gems” due to its unique play of colors that can range from blues and greens to oranges and reds. But what exactly is opal and what class of mineral does it belong to? Let’s explore the answers to these questions and more.

What is Opal?

Opal is a hydrated amorphous form of silica—or silicon dioxide (SiO2). It has a hardness rating of 5.5 – 6.5 on the Mohs scale. Opals are composed of tiny spheres of amorphous silica which diffract light, creating unique patterns that vary depending on where they were formed, making each stone unique.

What Classification is Opal?

Opal is not a mineral but rather a mineraloid. A mineraloid is an amorphous substance that forms in geological settings similar to those of minerals. The main difference between opal and minerals is that minerals are crystalline solids while opal does not have an ordered arrangement of atoms. Rather, opal consists mainly of hydrated silica and water.

Opals can also be classified as either common or precious opals. Common opals have a milky white color while precious opals are composed of tiny spheres which create a “play-of-color” effect under light, resulting in brilliant flashes of color that appear when the stone is moved or tilted. This type of opal is very rare and highly sought after for jewelry making.

How Is Opal Made?

Opal forms when rainwater containing silica seeps into cracks in sedimentary rocks such as sandstone or limestone over millions of years. The water evaporates over time, leaving behind deposits of silica in the rock which solidifies into opal when exposed to air. 

Is Opal Expensive?

The cost of an opal depends largely on its color, size, shape, clarity, origin, and quality. Generally speaking, common opals tend to be less expensive than precious opals but both can range from very affordable to incredibly expensive depending on the aforementioned factors. Prices can range anywhere from $10 USD for a small piece up to thousands if not $10,000+ USD for larger pieces with exquisite coloring or uncommon shapes.

The price range for opals can vary depending on their origin, color, size, clarity, and carat weight (the unit used to measure gems). Generally speaking, white opals tend to cost less than black or boulder opals due to their more common availability. Similarly, fire opals are usually less expensive than precious opals because they lack the play-of-color found in precious stones. On average, you can expect to pay anywhere from $2-$150 per carat weight for most types of opals; however prices can go as high as $1,000+ per carat weight for rarer varieties. 


Opal is one gemstone that truly stands out among others thanks to its fascinating play-of-color effect that varies greatly depending on where it was formed and how it was polished afterward. While there are two main types – common and precious – both come with their own unique appeal that make them desirable additions to any collection! Whether you’re looking for something affordable or luxurious, there’s sure to be an opal out there just perfect for you – no matter your budget! So if you’ve been wondering about this mesmerizing stone lately, now you know all about what it is and why it’s so special!