If you’re a gemologist, jeweler, have lapidary as a hobby, or you love collecting and buying rocks, minerals, or crystals, you’re in great company.
In this article, we discuss agate and opal in-depth. Read on to find out what these gemstones are and their differences and similarities.
We hope this article is a gem to you.
Let’s dig in.
Agate vs Opal (EXPLAINED)
What Is Agate?
Agate is made of banded microcrystalline Quartz, which is one of the two elements found inside chalcedony.
The banding patterns of agates distinguish them from other forms of chalcedony, such as Jasper and Onyx, which are not banded.
Agates come in multiple color patterns, such as white, blue, green, brown, pink, purple, gray, black, yellow, orange, or multicolored.
Even so, each agate is unique and has different types of banding.
An agate may have multiple hues of the same color.
A good example is blue lace agate which has dark and light shades of blue.
Also, agates may have different hues of many colors.
For instance, iris agate displays all the colors of the rainbow when adequately illuminated.
Another defining quality of all agates is translucency.
This means that light can pass through the agates, primarily through the edges.
The two main features that distinguish agates from other types of chalcedony are translucency and bands of color.
How are Agates Formed?
Agates occur as noodles inside the cavities of igneous and metamorphic rocks.
Agates are mostly found in volcanic rocks such as basalt.
Agates form inside these rocks or on the rocks.
This happens after the rock formation process is complete, following the solidification of the molten rock material.
During the solidification stage, when lava is cooling down, gases form bubbles inside the igneous rocks.
Ultimately, the bubbles solidify, leaving small cavities in the rock.
When water with a high concentration of silica deposits inside these cavities, agates form eventually.
As the water with concentrated silica (SiO2), commonly referred to as Quartz, seeps into the cavities of the volcanic rocks, it solidifies and becomes silica gel.
The silica gel deposits on the walls of the cavities and slowly crystallizes as the water evaporates.
Gradually, more silica gel is deposited into the cavities upon the previously-formed layers.
The new layers of silica gel build up horizontally or in concentric circles.
This build-up is a long-term process that happens over several years.
This means that each new layer of silica gel will have varying compositions of minerals and impurities, given that they are formed at different times of the year.
And this is why agates are banded.
Types of Agates
As previously mentioned, agates can be found in many natural colors.
However, agates are classified using more parameters than the bands of color.
As such, there are many types of agates available.
They include blue lace agate, fire agate, crazy lace agate, eye agate, agate geode, condor agate, Laguna agate, and fortification agate.
Sources of Agate
Other countries with large agate deposits are Botswana, Australia, India, Poland, Mexico, Brazil, Argentina, and Uruguay.
Now that you know more about agate, let’s explore opal in depth before discussing the similarities and differences between agate and opal.
What Is Opal?
Opal is formed when amorphous lumps of silica mix with water.
Scientifically, opal is silicon dioxide combined with water (SiO2H2O).
Opal is believed to have formed millions of years ago when silica-rich water seeped into the deep cracks of the Cretaceous layer of rock.
As water trickles down the earth, it collects silica from sandstones and leaks the silica-rich water into the cracks and voids on the rocks.
The voids are formed by natural faults such as tensional or compressional stress or by decomposing fossils.
Eventually, the water evaporates and leaves silica deposits in the cracks and voids on the rocks.
Over time, as the cycle repeats, opal is formed.
Types of Opal
Opal is separated into common opal (potch) and precious opal (noble opal).
Precious opal is the most beautiful and valuable opal because it has the feature called play-of-color.
This means that the opal can reflect and refract light to form a spectrum of colors that looks like a rainbow.
The spheres of transparent hard silica in precious opal are arranged in a systematic three-dimensional frame that allows the noble opal to reflect and refract light.
This phenomenon is known as opalescence and is only found in precious opal.
Popular varieties of precious opal include white opal, black opal, and boulder opal.
Common opal doesn’t have the play-of-color feature.
Hence, it’s less desirable and has fewer commercial uses compared to noble opal.
Most of the opal mined worldwide is potch.
On the other hand, noble opal is rare and accounts for about 0.25% of the total opal mined and is mainly found in Australia.
The other types of opal include Fire Opal, Girasol opal, and Peruvian opal.
Sources of Opal
Opal is found in plenty worldwide in Brazil, Ethiopia, Mexico, Peru, and Australia.
However, more than 95% of the total previous opal produced globally is found in Australia.
That said, let’s explore the differences between the two gemstones- agate vs opal.
While agate is formed by microcrystalline, cryptocrystalline Quartz, or finely crystalline, Opal is a noncrystalline form of the mineral silica.
Opal is an amorphous hydrated silica oxide.
Following the Mohs hardness scale (1-10), agate has a hardness of seven while Opal’s hardness ranges between 4.5- 6.5.
Agates belong to the hexagonal crystal system.
Since opal is noncrystalline and amorphous, it cannot fit under any of the seven crystal systems.
Opal is transparent to opaque, while agate is translucent.
The specific gravity of a gemstone is measured by the relative comparison weight of water.
This refers to how heavy the gemstone is in relation to water.
The specific gravity of agate ranges between 2.63 – 2.65, while opal ranges between 1.98 – 2.25.
Hence, agate has a higher density than opal.
Agate vs Opal: Similarities
Both agate and opal are cut and polished into cabochons to be used for decorative purposes in jewelry.
The two gemstones make beautiful ornaments such as bracelets and necklaces.
Agate and opal occur in multiple varieties depending on the dealers and the place of origin.
For instance, opal from Lightning Ridge, Australia is called Lightning Ridge Opal, while agate from rom Ojo Laguna, Chihuahua, Mexico, is called Laguna agate.
Most places that have large deposits of agate also have Opal.
These are places such as Mexico, Australia, and Oregon in the United States.
The reflective properties of both opal and agate are described as vitreous.
Having explored the properties of agate and opal in-depth and their similarities and differences, you certainly have more knowledge of the two precious gemstones.
When buying, collecting, or selling agate and opal, this article will come in handy as a resourceful reference.