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Is Opal a Mineral or Mineraloid? Learn How To Classify Opals!

is opal a mineral or mineraloidIn the fascinating world of gemstones, there are two types: organics and minerals. However, these are two broad categories within which smaller subsets exist. One example is mineraloids, a mineral-like specimen without a crystal structure. You might wonder, is opal a mineral or a mineraloid?

The two are quite similar, but they do have a notable difference in their chemical composition. One has a crystalline structure, and the other doesn’t. 

Opal is unique because it exhibits play of color, a mesmerizing optical effect where vivid colors blaze through the stone. Without a doubt, opal is one of the most fascinating gems in the world, but is it a mineral?

Join us on a geological expedition to discover the difference between minerals and mineraloids and in which category opal belongs.

What Is A Mineral?

If a gemstone has a crystal structure, it’s a mineral. A crystal structure develops when certain gems form. Geological influences like heat, pressure, and host rock size shape the gemstone as it grows. To classify a specimen as a mineral, the atomic structure must meet the following criteria:

  • The specimen must be inorganic (meaning it’s not from a live organism like pearls or amber).

  • It must naturally occur.

  • There must be a definite, solid chemical composition. 

  • And finally, it must have an ordered atomic structure.

All of these factors mean that a mineral has a crystalline structure that isn’t amorphous. The internal structure of mineral gems is atomic (ordered) according to acceptable ranges.

Examples of minerals include diamonds, emeralds, garnets, and rubies.

mineral crystal - topaz

However, not all gemstones have a crystalline structure, which leads us to mineraloids.

What Is A Mineraloid?

Based on appearances, minerals and mineraloids overall look like gemstones. It’s difficult to recognize a mineral from a mineraloid because their chemical composition isn’t visible. 

Closer inspection of the specimen will reveal the geological influence on the gem. So, what are mineraloids, and is opal one of them?

Minerals are similar to mineraloids, except the latter doesn’t have a crystal structure. 

Mineraloids have an unorganized atomic structure. What does that mean? Essentially, that it cannot form crystal structures under any circumstances. Mineraloids are amorphous, meaning they don’t have crystallinity. As such, mineraloids exhibit atomic structures beyond accepted mineral ranges. 

opal is a mineraloid
Like minerals, mineraloids occur naturally and grow in specific geologic conditions.

Opal is a mineraloid because it is amorphous hydrated silica. Despite its mineral-like properties, opal’s chemical structure is hydrated with varied water concentration. Because of this variance, the stone’s structure isn’t fixed or organized and thus, qualifies as a mineraloid.

Here’s a brief list of mineraloids:

  • Pearl - Certain gemstones are organics because they come from a living organism. Pearl is a mineraloid bound by organic material.

  • Amber - Born from fossilized plants, amber lives in sedimentary rocks and has a non-crystalline organic structure.

  • Obsidian - This volcanic silica glass does not have an ordered crystal structure.

Additional examples include shungite, jet, and tektite.

Can You See The Difference Between A Mineral and Mineraloid?

It’s not always prevalent to the eye whether a gemstone is a mineral or mineraloid. Fortunately, most of the popular variations are widely recognized and acknowledged among gemologists. But what about rare gems? Can you tell the difference by looking at them?

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These specimens require analysis at a lab. Fortunately, there are two recognized systems for classifying gems as either minerals or mineraloids. 

The International Mineralogical Association (IMA) has approved 5,230 minerals from the 5,300 known mineral species, according to the following classification systems:

But what if you’re not a scientist or geologist? There are a few recognizable features for identifying minerals and mineraloids.

  • For minerals, prominent features include a definite crystal structure that displays luster, cleavage, magnetism, and hardness.

  • Identifying mineraloids works in the exact opposite way. If a specimen doesn’t have a crystalline structure and the associated features listed above, it’s likely a mineraloid.

So, why is opal a mineraloid? 

Is Opal A Mineral Or Mineraloid?

We’ve explained that opal is a mineraloid, but why? What chemical features in the gemstone’s atomic structure make it a mineraloid?
While the Dana system constitutes opal as a true mineral, it lacks a crystalline structure. Why the confusion? Pure opal has silica spheres that appear to be arranged in an organized pattern under an electron microscope. When light enters the stone, and the spheres diffract it, a vivid color scintillates through the opal. This effect is called play of color, and is caused by the perfectly aligned silica spheres within the opal’s chemical structure.

However, most common opals have disordered silica spheres, which cause cracks in the internal structure, called crazing. The once aligned spheres are now unpredictable and disordered. 

As a result of this process, opals are considered a mineraloid because their original intact silica composition is disrupted. And of course, the primary reason opals are mineraloids is because they lack a crystalline structure.

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Opal: A Striking Mineraloid!

Mineraloids are impressive gemstones because of their misaligned, non-crystalline chemical structure. Unlike most translucent gems like diamonds or sapphires, opals are opaque and non-vitreous. 

Mineraloid opal is a fascinating specimen grown from lumps of silica spheres and the perfect combination of silicon dioxide and water. Opal grows in the earth’s crust from bubbled up geothermal baths that dried over millions of years. Wearing an opal mineraloid is like wearing a piece of ancient history! 

Ready to wear your own beautiful mineraloid? Shop our opal collection, including opal jewelry, Australian opal, uncut opal, and more!

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