Unlike other minerals and gemstones, the mineraloid opal is created from the interaction of water and silica. On average, opals contain about 10% water, but it varies depending on the specimen that you own. There are some variations in the water content of this mineraloid. The value of opal depends mostly on the uniformity of the silica spheres inside.

The consistency of the size of the spheres, as well as the gaps between them, determines who opal will reflect light, and how well it can showcase its natural color. Take note that there is precious opal and non-precious opal. Common opal is often brownish and is translucent, with hints of orange. Despite it not being the most valuable of opals, common opal is still beautiful and looks fantastic in decorative and jewelry applications.

Opal is geologically formed in ellipsoidal formations on boulders. On average, miners can recover specimens measuring a few centimeters across, but there have been cases when the opal is a massive three meters in size. There may be random distributions of opal depending on the mine and the conditions. Because it is not the most durable of mineraloids, it may be difficult to recover all of the opal without cracking the samples.

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How many types of opal are there?

There is a massive variety of opals to be had, and each opal has its unique quirks that make all of them great for collecting as jewelry or loose stones:

Black opal

Black opal has a predominantly dark body with inclusions that can range from blue to orange to yellows. This type of opal is normally found in New South Wales, and it is the most popular of all the opal types. The name is also confusing for some because “black opal” does not necessarily mean that the entire gemstone is black. What the name implies is that the majority of the body of the gem is dark, but there may also be other colors. Black opal is going to be darker than white opal, though both may feature imperfections or inclusions. The spectrum of colors present on a black opal specimen will not change its type or category.

White opal

Also called milky opal, this is another popular type of gemstone that is mined more often in the southern region of Australia. Due to its milky appearance, it is not able to feature the depth of its colors the way other opals do. High-quality white opal still fetches a lot of money on the market even though color depth is not this gemstone’s strong suit.

Boulder opal

Boulder opals are often mined from ironstone deposits in Australia, specifically in Queensland. The seam between the boulder opal and the ironstone itself is often thin, so there is a considerable risk that you are going to crack the opal in trying to extricate it from the boulder. What miners do is they remove the opal along with a bit of the boulder itself. The opal forms between the cracks of the host rock, so you can imagine how challenging it is to remove the opal when it is bound tightly to the host boulder in all directions. Boulder opals are usually dark, with amazingly deep color.

Crystal opal

Crystal opal is distinct from the types we have discussed earlier because it can either be wholly transparent or just partially transparent. If you hold a crystal opal to eye level, you will be able to see through its crystalline structure. Sometimes, crystal opals may be darker in color, but they still retain their transparency. This falls into the subtype of black crystal opal. When a milky hue somehow reduces the transparency, the gemstone is classified as white crystal opal. The color and tint of the gemstone often precede its type, so there won’t be any confusion.

Fire opal 

Fire opals are warm gemstones that have scores of reddish tones and hues, including red, yellow, and oranges. These are the brightest opals when held up to natural light, and you will even be treated to an otherworldly show of green flashes when the light hits the body of a fire opal. The best fire opals have been mined from Mexico, and thus they are aptly named Mexican fire opals. In some cases (when conditions allow), fire opals are cut entirely away from their host rocks if they can waistband the pressure and cutting method. Again, opals have water content and are mineraloids, so they are not as hard as say sapphire, or diamonds.

Girasol opal

Girasol opal is a milky type of quartz that is mined from Madagascar. It exhibits a high level of the asterism. Asterism or the star effect is observed when you hold up a gemstone to the light and rotate it. You will see a star formation inside as light is bounced around the crystalline structure of the gemstone. Girasol opals are often mistaken for fire opals.

Peruvian opal

This is a semi-opaque type of opal that usually includes the host matrix or rock from whence it comes. Blue opals are usually Peruvian, but some have also been mined from the United States, in Nevada.

 

Where is opal found?

The majority of the world’s opal supply is from the subcontinent of Australia. In recent years, the needle of supply has moved in favor of other countries, but Australia is still delivering substantial lodes of opal consistent. Other parts of the globe are attempting to catch up: Brazil, Indonesia, the United States, Mexico, and Ethiopia have also been mining opal continuously in the last decade.

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How much is opal worth?

Due to the diversity of opal types, it can be challenging to gauge the value of a sample immediately. For decades there was no single standard for pricing opals, even in Australia, where more than 90% of the world’s supply of commercial-grade opal is mined.

The largest and rarest opal in the world currently is the Fire of Australia, which is priced at more than half a million dollars. This specimen of opal is as large as a football. Depending on the grade of the opal, it can be as cheap as $10 per carat, or as expensive as $6,000 if the stone is unique and of high quality. Black opal mined in Australia can cost $30 for half a carat, and $1,500 for a whole carat. If the opal exceeds four carats, the whole gemstone can cost a stunning $50,000.