Agate is one of rockhounders' most beloved stones. It comes in a variety of vibrant colors and intricate patterns, and can be easily found in the gravel of shorelines, rivers, or fields.
The name "agate" was given to the mineral by Theophrastus, a Greek philosopher, who, in turn, derived it from the Italian River Achates (now the Dirillo), where those stunning gemstones were found.
Agate is a transparent or translucent variety of microcrystalline quartz. The term "agate" also covers nonbanded chalcedony containing colorful impurities of minerals or water or gas bubbles.
Agate is relatively high on the hardness scale and is resistant to acids. Like other forms of chalcedony, agate is roughly 90% quartz and 10% moganite. Both quartz and moganite share the chemical formula of SiO2, with the only difference being their respective crystal structures.
Agate Formation
Agates tend to form within volcanic basalt or other extrusive igneous rocks. They can appear as veins, nodules, or casts of organic materials, such as wood or animal remains. As molten lava cools, gases trapped within rise toward the Earth's surface, creating cavities on their way up. Those cavities gradually fill with silica-rich fluids that, over time, deposit tiny fibrous crystals along the cavity walls.
As the crystals grow, mineral impurities become trapped within the bands, creating contrasting rings typical of agate.
Agates are harder than their host rocks, which eventually break, releasing the agates.
Guide to Agate Identification
Agates possess unique characteristics that allow them to be distinguished from other minerals with fair certainty. Below, we will list several such characteristics accompanied by the description of tests you can use to understand whether the mineral you have found is indeed an agate.
Agate Hardness - The Knife Test
Agate's hardness is pretty high -- between 6.5 and 7 on the 1-to-10 Mohs scale (with 10 reserved for the hardest mineral). It cannot be scratched by a steel knife.
To perform a knife test on a mineral, gently scratch its surface. If the knife does not leave a mark, it is likely an agate you are looking at.
If you see a mark left on the rock, try moving it with your finger to ensure it is indeed a scratch and not some unrelated material from your blade.
Agate Transparency - The Light Test
Agate is known for its translucent to semitransparent nature.
Distinguishing agate from jasper can sometimes be tricky. The Light Test will help tell them apart. Place a light source behind the stone and observe the stone's physical properties. The stone is likely agate if the light passes through, even partially. If the stone remains wholly opaque and the light does not pass through, it is likely jasper.
Even though most agates are translucent, certain exceptions do exist. One such exception is Lake Superior Paint Agate, which is not translucent. In such cases, make sure you check the banding.
Agate Color
The color of an agate is primarily influenced by the embedded minerals.
Iron oxides tend to be the most frequent inclusion. Other minerals found in agates are manganese, chromium, celadonite, nickel, copper, and hematite. Each comes with its own unique color contribution, including multicolored patterns, white, gray, orange, brown, pink, purple, black, and yellow. Green and blue tones are particularly rare.
Some of the places where rare blue varieties are found are:
- Near Ellensburg, the State of Washington (home to Ellensburg Blue Agate);
- Sweet Home, Oregon (home to Holly Blue Agate);
- Polka Dot Agate Mine, Central Oregon (home to Blue Ice Ocean Agate); and
- Newport, Oregon (home to Blue Beach Agate).
Agates with "non-standard" inclusions, such as dendrites, sagenites, moss, and plumes, are especially rare and sought after.
Agate Banding
Agates can be distinguished from such look-alikes by their uneven banding. Concentric bands run through the stones and can be thin or thick. Unbanded specimen is just calcedony, but rock collectors use the term "agate", also, for dendritic, moss, flame, and plume form of chalcedony.
Agates always have banding, while jasper rarely possess any type of banding but can exhibit spotted or striped patterns.
To better identify banding of the rough rock, spray some water on the suspected agate to make the lines stand out.
Rocks and Minerals Similar to Agate
Some minerals (especially jasper, quartz, and nephrite) can be easily confused with agates due to their similar appearance.
Below, we will list several distinguishing features:
- Jasper: Jasper is usually darker and opaque, with spotted or striped patterns. It typically comes in yellow, brick-red, brown, and mottled-green colors.
- Quartz: Quartz is generally translucent. It tends to have a more uniform appearance - no bands or multi-colors.
- Nephrite: Nephrite usually comes in shades of green, with no banding. It is also softer than agate.
When identifying an agate, keep the entire list of distinguishing attributes in mind and use them in combination. Reliance on a single characteristic may be easy, but it is also deceiving. Imagine encountering a banded jasper! Yeah, I got myself confused, too.
Value of Agates
Most agates have low monetary value, but some unique and large pieces might be worth hundreds of dollars. Lapidary work that includes shaping, polishing, and engraving increases the agate's value dramatically.
Agate is used to make jewelry, beads, vases, ornaments, and other stunning artworks.
Lapidary Use
High scratch resistance, bright colors, and exquisite banding make agate the most popular lapidary material. Specimens with banding are often cut to expose the beautiful pattern and then face polished.
Agate is also an excellent tumbler material and great for making cabochons.
Where to Find Agates?
Agate is a gemstone that occurs in numerous locations worldwide. In the United States, it is primarily found in Western states known for significant volcanic activity, such as Arizona, California, Idaho, Montana, Oregon, and Washington. In the U.S. Midwest, one of the most notable agate locations is the shoreline of Lake Superior in Michigan.
Agate is especially abundant in river and creek gravels along the Pacific Coast of Washington, Oregon, and California, as well as in fields and mountainous regions. It is also commonly found in the cavities of igneous rocks and is a frequent component within thundereggs.
