How to Identify Agate

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Agate, a variety of chalcedony, is a popular gemstone due to the characteristic bands that form inside of the stone and come in many colors, from deep reds and soft pinks to brilliant greens, deep blues, and everything in between. [1] Agate is also popular for gem collectors, both amateur and professional, because it is relatively easy to find and polish. If you have a stone that you think could be agate, there are ways to quickly check, such as comparing your sample's appearance to known agate or testing for the typical translucence of an agate stone. If you end up with a bit of agate in your collection, your stone can be polished up and made into something truly unique and beautiful!

Things You Should Know

  • Agate has multicolored bands that can appear as concentric circles or horizontal layers. [2]
  • The inside of an agate stone will be smooth and feel glassy.
  • When you hold agate up to light, it's transparent.
Method 1
Method 1 of 2:

Checking the Physical Properties of Your Sample

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    1
    Feel the rock's weight in your hand to estimate density. Agate is a relatively dense stone, so it should appear to weigh less than it actually does. Hold the stone you think is agate in one hand, and then hold a similar sized stone that you don't think could be agate in your other hand. If the stone you think is agate feels heavier, you may be correct. [3]
    • This is a relatively inaccurate way of estimating density. If you want to be more exact, you can calculate the density of the stone using a water-displacement method. Agate has a density of 2.6 to 2.64 g/cm3. [4]
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    2
    Feel for a waxy or glassy texture on the inside of the stone. If you can break open your sample with a hammer and chisel, or if it is already broken, rub your finger along the inside of the stone where the bands of color are. Agate should feel waxy or like glass. [5]
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    3
    Compare the color and appearance of your stone to known agate samples. To do this, you will have to see the inside of the stone, so you may have to crack it open with a hammer and chisel. Mineralogists characterize agate by the stone's multicolored bands, which can either form concentric circles that grow towards the center or, more rarely, horizontal layers. [6]
    • Other types of chalcedony stones (jasper, chert) display banding, so you may have to perform other tests in order to make sure what you have is agate.
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Method 2
Method 2 of 2:

Determining the Transparency of Your Sample

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    1
    Open the stone with a hammer and chisel if your sample is not already broken open. The rough outside of agate will block all light, so you'll have to get it open in order to test the transparency.
    • To crack a bit of stone, place it on a solid, stable surface. Put the pointed end of a chisel up to the stone, and then hit the other end of the chisel with a hammer. It may take a few tries!
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    2
    Hold your stone sample up to a light source, such as a lamp or flashlight. One of the defining features of agate is its translucent quality. To determine how transparent your sample is (which is also known as its diaphaneity), position the stone so that it comes between the light source and your eye. [7]
    • Agate does not need to be polished for it to be translucent, so this quality should be apparent in its natural state.
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    3
    Look to see how much light from the source passes through the stone. Agate is translucent, which means that only some of the light passes through. [8] When you hold the stone up to a light source, the colors of the agate should shine a little and become more clear.
    • If no light shines through, then the stone is opaque. This indicates that your sample is not agate, but most likely is jasper. [9]
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    4
    Test transparency by holding your agate over an image on paper. Because agate is translucent, when you hold it over an image of something, the picture should be able to be seen, but it should be slightly blurred. [10]
    • If you can clearly see through the stone, then your sample is transparent. This may indicate that what you have is not agate, but a different type of quartz crystal.
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Expert Q&A

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  • Question
    What causes agates to form?
    Sam Lagor, MSc
    Geologist
    Sam Lagor is a Geologist with over eight years of experience. He specializes in engineering geology (dams, bridges, and tunnels) and mineral exploration (gold, lead/zinc, andindustrial minerals). Sam holds a BS in Geology from St. Lawrence University and an MS in Geology from The University of Vermont. He is also a member of the Geological Society of America and the American Institute of Professional Geologists.
    Sam Lagor, MSc
    Geologist
    Expert Answer
    Agates form in void spaces underground, which are created by limestone eroding away. Sometimes, really hot hydrothermal water flows up into these void spaces as it looks for low-pressure environments closer to the crust—this dissolves certain minerals out of the rock. As the hydrothermal water flows up, some of the dissolved elements precipitate back onto the walls. The precipitation of silica quartz forms the different layers of agate.
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      References

      1. ↑ Sam Lagor, MSc. Geologist. Expert Interview. 7 October 2021.
      2. ↑ https://www.gemsociety.org/article/agate-gem-information/
      3. ↑ https://sciencing.com/identify-unpolished-agates-6180873.html
      4. ↑ https://www.mindat.org/min-51.html
      5. ↑ https://sciencing.com/identify-unpolished-agates-6180873.html
      6. ↑ https://geologyscience.com/gemstone/moss-agate/
      7. ↑ https://www.gemsociety.org/article/gemstone-transparency/
      8. ↑ https://www.minerals.net/gemstone/agate_gemstone.aspx
      9. ↑ https://rocktumbler.com/blog/what-is-agate-jasper-chalcedony/
      1. ↑ https://rocktumbler.com/blog/what-is-agate-jasper-chalcedony/

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