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How to Cut & Polish Thundereggs: Follow These 6 Simple Steps

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The thunderegg is truly a mysterious beast, from the way it forms to the way it looks. To reveal the mystery around how to cut and polish a thunderegg, we have to catch a glimpse of how they form, and then construct this history for each of the thundereggs we cut.  

The goal of cutting a thunderegg is to reveal and highlight the inner geometry by cutting through any air pockets that exist and perpendicularly through the mineral layers. To accurately do this, one must estimate the inside layout based on how the egg formed. After cutting, sand and polish the flat surface with cerium oxide for the best shine.

This article will explore the sources of these ridges on thundereggs and what they tell about the contents inside, and how to best cut and polish them after we find out.

If you are interested in checking out the best tools for cutting rocks you can find them by clicking here (Amazon link).

How to Cut & Polish Thundereggs
How to Cut & Polish Thundereggs

Step 1: When You Find Them, Mark Them

If you have found the thundereggs you want to cut already, then this is step is impossible for you, so skip on to the next one. However, if you plan to go looking for them, this step is a strange but vital step to getting the best cut.

To understand this rule, we need to know a little about the end of the forming process. Thundereggs are very dependent on gravity, and guessing what is on the inside is made much easier by knowing the orientation that they have formed in.

So, when you pick the eggs out of the host rock, the best practice is to put an “x” on the horizontal position of the egg, and a “-“ on the vertical position. Using these, you will have an easier time knowing where to cut.

The amazing agate and quartz inside have been transported there while in solution with water. The water picks up the silicon from surrounding rock structures and transports it to the vicinity of the already hollow thunderegg (we will see how it becomes hollow later). It then is able to diffuse through the pores of the soft igneous rock on the outside into the inside.

Given enough time, it will crystallize on the inside surface of the hollow egg. Different minerals contained in this solution will create different colored layers inside the egg.

Regardless of the color, the minerals in solution with water have a tendency to follow gravity to the bottom of the hollow hole. Therefore, marking the position in which you found it will help make sure you’re informed enough on each thunderegg to make the cuts you want. 

It is possible that rock movements have changed the orientation of the thunderegg so that the orientation in which you find it is not the position in which it formed. In this case, try to find the center of gravity of the thunderegg by setting it in a few different positions and seeing if it has a tendency to roll in a certain direction. Remember that the heaver portion of the rock will end up at the bottom if it can freely roll.

Step 2: Choose Your Tool

Now that we hopefully have the orientation of our thundereggs, we can prepare the other side of this operation.

If you have found, bought, or broken geodes open, you’ll know that it is generally regarded just fine to break a geode with a chisel and hammer. On the contrary, thundereggs are often much thicker and have a distinct difference from geodes that makes a chisel an inadequate tool for breaking them.

Thundereggs form in layers of different types of stone. This geometry makes it likely that, when hit with a chisel, the egg will break along the layers rather than along the middle. For this reason, we should use a saw instead, but what type of saw?

Because you may not be looking to go spend the money for a top-of-the-line rock saw, you may be wondering if the saw you currently have is good enough. Here are some things to take into consideration:

  1. The saw must be meant for some type of stone (usually means a diamond tipped blade).
  2. The blade diameter must be at least three times the diameter of the egg you are cutting.
  3. You must have a stabilization strategy.

The last of these is possibly the most important because it can have severe safety implications. You must have a way to make sure the egg does not move an inch during the cutting process. Some drop saws cannot fulfill this requirement because the egg has 360 degrees of rolling possibilities. Clamps and angled supports are good strategies, and hands are not!

Step 3: Classify Each Thunderegg

Not all thundereggs are created equal, at least when it comes to the orientation of their insides. The differences in this area also come from the formation process. In this case, we need to focus on the water which is trapped inside as the lump of cooled magma sinks back into the deeper layers of the earth’s crust. 

As this solid sinks deeper, the water boils into steam, and then the pressure ramps up as the temperature does. Depending on the thickness of the outer layer, this can sometimes end up in a bomb-like explosion. Other times, the steam finds a few specific routes out of the rock (like the filling of a thin dumpling if you do not bite hard enough). 

These routes that the steam takes out classify the thunderegg. If only two or three routes are found and they are all at the top, then the silicon-saturated water which enters later will collect at the bottom. If there are many found and they are all over the stone, then it will form symmetrically, built around a central air gap. 

You can often see where the steam was released from the ridges on the stone. If there are too many ridges to accurately guess the steam’s path, this is classified as a thunderegg with “flow bands.” Flow bands are ridges all facing the same direction.

Step 4: Know Where to Cut

Now we know everything we can about the thunderegg, so we need to decide where to cut. Here is where you can use your creative freedom and cut based on what you want the product to look like. 

Here are things to keep in mind:

  1. Larger chunks of agate may be found in areas with fewer ridges.
  2. Cutting perpendicular to the layer structure will show off the geometry of the layers ideally.
  3. It is recommended to cut across the air pockets.

With this information in mind, feel free to change the direction of the cut of each thunderegg to get the best feeling for what it looks like to cut through, across, or perpendicular to layers and air pockets.

TIP: Not sure how to cut the rocks? Check out this complete step-by-step guide on how to cut rocks with a hammer and chisel or alternatively the article about the best tools for cutting rocks:


Step-by-Step: How to Cut Rocks with Hammer and Chisel


What Can I Use to Cut Rocks? These 5 Tools are the Best!


Step 5: Sanding

Thundereggs are a unique polishing case in the lapidary realm because the main draw of a thunderegg half is a paper flat edge. With a typical stone polishing tool such as a Dremel drill, you cannot get a large enough attachment to get the flat edge you want. For this reason, it is best to use an industrial sander. 

Industrial sanders are made to smooth either rock or wood. They are up to a foot in diameter and sand using friction, either vibrating or rotating. If your saw does a good job making a flat surface as it is, I recommend using a wood sander.

You will want to change the sanding grit 3-4 times throughout the process, just like with tumbling or using a Dremel. In this case, they are sheets of the same diameter as the power tool, and a good progression to use is:

  1. 120 grit
  2. 400 grit
  3. 800 grit
  4. 1200 grit

The same rules for sanding apply for sawing – make sure you have the rock clamped in so that it cannot move, even if you apply a good amount of pressure to the open edge. With a wood sander, you will have to apply a good amount of pressure and maybe spend a little more time on the rock face, but it is gentler and ends up with a smoother product.

TIP: Dremel drills are perfect polishing tools. Find out more about polishing rocks with dremel drill in the article below:


How to Polish Rocks With a Dremel Drill? Follow These 4 Steps


Step 6: Applying a Polish

Once the face’s surface is soft to the touch, it is most receptive to a good polishing compound. Because the minerals in thundereggs are always in the quartz family, there is a usual suspect for polishing which you can count on to polish all your thunderheads. 

This compound is cerium oxide. It is most known for its use in polishing glass (remember, class contains mainly silicon oxide just like quartz or agate).

You can often find a soft felt attachment for sanders which is used for applying polish. If you cannot find this attachment but you have a Dremel, you can use that instead for this step. Just dip the attachment into the cerium oxide polish and slowly buff out the agate portions of the thunderegg. Keep following the process until there is no noticeable change in the rock after a buff. 

Once you have the final, polished, and geometric thunderegg half, find an adequate place to display your work. Then check out some more rockhounding content and tutorials on HowtoFindRocks!

TIP: And it’s rockhounding time now! So let’s find your own amazing thundereggs. Check out the articles with the best rockhounding locations in the United States for each state in the section below:


Best Rockhounding Locations by Each State in the US