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How To Cut and Polish Thundereggs (Step-By-Step Guide)

what is a thunderegg vs geode

So, you’ve got yourself a nice thunderegg sitting on your desk, but you’re not sure how to turn it into a display piece?

You’re in the right place! We’re going to dig in and show you how to cut and polish thundereggs, from start to finish!

What is a Thunderegg?

Thundereggs are a geode variant that are filled instead of having a hollow pocket lined with crystals in the center. They’re best thought of as a unique, single mineral formation since the minerals that are contained inside vary.

Thundereggs are usually some form of cryptocrystalline quartz, chalcedony and agate are among the most common minerals contained inside of them. Jasper and chert are also found in many of them. They’re a fascinating subject, and among the most popular mineral displays.

The takeaway is simple: thundereggs are a round mineral formation that has been filled entirely. 

What You Need to Cut and Polish Thundereggs

You don’t need a lot of specialized equipment to cut thundereggs. We’ll go over a couple methods of getting most of the work done, but you’ll still need some tools and PPE.

The hardest thing to improvise is your rock saw. A rock saw with an appropriately sized blade for the thunderegg does best, but you can use a cheaper tile saw. In my experience, they tend to come with a wide kerf blade and aren’t as steady as purpose-built rock saws.

If you’re a complete newbie, I recommend finding a cheap tile saw and putting a lapidary blade on it. I’ve personally used a very cheap tile saw with a lapidary blade to split open nodules in the paste. Most diamond blades will work as long as they fit.

A slab saw is still better, and your local lapidary club will usually have access to one.

You also need two pieces of PPE:

  • Safety Glasses- ANSI-rated, non-negotiable. While it’s not a common problem, fractured stone can spit out of a saw at a high rate of speed. Keep your eyes safe, it’s a very minimal effort required to save your vision.
  • Particulate Respirator- A dust mask can work if you’re outside during a breezy day but a respirator is best. Silica dust isn’t just irritating, it can kill you. If you’re on the fence… read about silicosis and make an informed decision on your safety.

You’ll want all of the following:

  • Superglue or Epoxy
  • A bit of scrap wood
  • Flat piece of glass, smooth tile, or hardwood
  • Sandpaper in various grades to ultrafine
  • A bench polisher, Dremel, or flex-shaft
  • A safe work area
  • Tile or slab saw
  • Cerium oxide or preferred polishing compound

While most of the work can be done with things found at the hardware store, slicing the egg open will require some sort of saw. Thundereggs 1” or under can be cut with diamond cut-off wheels in a Dremel or flex-shaft but the cut will be rough.

Use the best tools possible. You can create a great specimen with just a wide tile saw blade and sandpaper, but you’ll also spend several hours longer than necessary.

how to cut and polish thundereggs

How To Cut and Polish Thundereggs

1. Prepare Your Specimen

To start off, you need to make sure that your specimen is ready for the saw.

Diamond saws usually won’t cut your hands, but it’s still not a great idea to give yourself an impromptu manicure by holding the egg directly. If you’ve never worked with a lapidary or tile saw, I don’t recommend running this one freehand.

I like to chip out a bit with a chisel before cutting. Just a small, ¾”-1” score will help guide the blade initially but it’s not necessary.

For best practices, you’re going to want to glue your thunderegg to the scrap wood mentioned above. You can glue a piece of wood on both ends as well, which helps you stay steady on the blade when cutting.

Use whatever your preferred cleaning method is and glue the stone in a way that you can access the piece you want to cut. If your saw has a guide, you should make sure that your mounting method is compatible.

High-end lapidary equipment will usually have a vise or other way to affix your specimen included. Use those when appropriate, it’ll save you the work of removing the glue later.

With the specimen cleaned and affixed, you’re ready to get to work!

2. Make Your Slice

Using a rock saw or tile saw isn’t overly difficult, but there are a lot of pitfalls that you can run into.

The most important thing to do when you’re cutting is to make sure you have appropriate coolant. In tile saws, this is usually water, but many lapidary machines use oil. Never cut a stone dry, even outdoors. The heat can ruin the blade, damage your specimen, and send huge plumes of toxic silica dust into the air.

If the saw is variable speed start it low. Less speed means less heat, leading to fewer problems but a slower cut.

Let the saw do the cutting. It’s human nature to force tools, but this never works out well for abrasive cutting. Putting extra pressure on the blade won’t make it cut faster, you can’t just shove harder and remove more material.

Light pressure and low speed are your best friends when cutting stones. Start low and only ramp things up if you’re not making any progress.

Thundereggs are usually thick and always hard. It may take a couple of minutes for an egg you thought was small.

The slower and straighter you cut, the less time the rest of the process will take. A bad cut with a rough blade can take hours to sort out.

I’ve heard varied opinions on pushing versus pulling the stone through the blade. My personal strategy when cutting nodules or thundereggs is to push if the stone is larger (2”+) and to pull if I’m freehanding the cut.

Keeping very light pressure is important for the end of the slice. The stone can react violently if you’re pushing hard and it breaks through. This can include unsightly fractures or bits of the matrix “blowing out” and removing chunks of material.

It just makes a mess. Trying to hurry up lapidary work invariably makes more work. An extra minute for the slice can save hours of sanding.

With a light touch, good mounting, and some luck you’ll have a perfectly cut nodule. Now it’s time to make it shine.

You can break the bond of the glue or epoxy with acetone if you have trouble removing it. Any extra glue bits can be brushed off with a wire brush and a little bit of solvent as well.

3. Surface Sanding

If you have access to a flat lap, you’re good to go. Just run the piece on the lap through the grits. I generally start at 220 and work to 1200 with a flat lap. I only use one intermediate grit between those, but I strongly suggest going through smaller increments if you’re a newbie. It’s much easier to correct errors that way.

Read More: How To Polish Rocks With Sandpaper? (Step-by-Step Guide)

For those of us who don’t have access to a flat lap, we’ve got two different methods for sanding.

The first is what most people would consider the easy way: mount a powered sander in some fashion and use it to sand. I don’t recommend doing this. It can shave some time but it’s… a bit outside of OSHA regulations.

If you choose to go down that route the only advice I’ll give is to make sure the sander is 100% solid wherever it’s mounted. It’s not a smart way to do things, in my opinion, and your own personal workshop and toolset will vary too much for me to give advice on mounting.

It doesn’t save as much time as you’d think either. Hand sanding is your best bet if you don’t have access to a flat lap.

The safest way to do things is to grab your smooth bit of tile, glass, or hardwood and some masking tape with sandpaper.

You’ll want to cover an area at least three times as wide as the widest point on your stone. Five times is much better, but not always possible depending on the size of the stone.

Now place the stone cut side down on the sandpaper. Apply light pressure and begin sanding the stone.

Learn to use a figure-8 pattern. It takes a bit more muscle memory than just making swirls, but it leads to a flatter result. Most people will have differential pressure when making “swirls” but a figure-8 pattern solves that issue.

If you really want to be thorough then you should rotate the stone 90° every five to ten seconds.

Keep going until the face of the stone appears uniform. At lower grits you’ll still have scratches once the stone has reached that point, and you’ll need to remove them with the next grit.

Then, strip off the sandpaper and place the next grit down. You can do this in a couple of ways. I recommend something like the following grit progression for someone without much experience sanding:

  • 220 grit
  • 400 grit
  • 600 grit
  • 800 grit
  • 1200 grit
  • 1500 grit
  • 2000 grit

And so on if you want to go higher. 

This is a time-consuming process, but it’s easier to go back and fix mistakes if you have smaller differences between finishes.

If you have experience sanding things to a fine finish, you can probably get away with fewer grits, just be aware it takes way more time if you have to go back to finish scratches. My personal progression is only three steps before polishing but going through fewer grits doesn’t necessarily save time.

Keep going through your grit progression until you’ve achieved a scratchless finish at your highest grit. If you’ve gone to 2000 grit or higher the stone will look “polished” but we’ve still got one more step to go.

4. Polishing the Face

A lapidary polishing wheel is best here, but I’ve used everything from a charged rag to a modified single-speed bench grinder for final polishing. You can even use a car buffer if you’ve got a strong vise and a large thunderegg.

For thundereggs, I recommend using cerium oxide. It’s a great polish for agate, jasper, and other hard cryptocrystalline materials.


For hand polishing, get your rag or chamois a bit wet, poke it in the powdered cerium oxide, and get to work on the surface. You’re looking at a lot of work, but it’s doable.

For those less inclined to sit around rubbing rocks for an hour with a rag, a Dremel is a good tool. You can use the felt polishing wheels that often come in tool kits and a low-to-moderate RPM to get your polishing done. 

Move the felt in small circles and cover the entire face of the stone. A microfiber cloth can wipe off excess polish and let you know if you’ve achieved your final look.

Grinders and bench polishers are also great. Again use a felt or cloth wheel charged with cerium oxide or another hard stone polish. Be careful with single-speed grinders, they often run at high RPM and will tear the stone out of your hand. 

Motors meant for polishing are a little less problematic, but the same safety rules apply.

There are as many ways to polish a flat surface as there are lapidaries, but we’re all after the same goal. A stone with a high polish should have a “wet” look.

The important things here are the polish and the soft surface charged with it. All methods achieve the same final look, but power tools get you there a lot faster.

Once the thunderegg is polished, all you’ve got to do is find a great place to display it!

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