Make Your Own Fossil: a Cement Ammonite Paperweight

I've always been fascinated by fossils.

Think about it. An animal lived in an ocean that no longer exists, died, and left a perfect copy of itself in stone. That stone then sat in the dark for millions of years, through ice ages and the rise and fall of the dinosaurs, until someone split it open and the shape saw daylight again.

Ammonites are my favourite. Their spiral shells are so precise they look engineered, with ribs curving in perfect rhythm around the centre. These animals filled the oceans for more than 300 million years before vanishing at the same time as the dinosaurs.

I wanted one on my desk, somewhere I'd see it every day. Good fossils aren't easy to come by, so I decided to make my own, and to make it the same way nature does.

Nature needs millions of years. I needed a 3D printer, 300 grams of cement, and a bit of patience.

The idea is simple. I 3D printed two things: an ammonite shell to use as a stamp, and a hexagonal tray to use as a mould. The tray is filled with wet cement, and the printed ammonite is pressed into it, just like a shell sinking into soft seafloor mud. Once the cement firms up, the stamp is lifted out. What's left is a solid cement hexagon with a detailed spiral fossil pressed into its face, with rough, broken edges as if it had been chipped out of a cliff. A few washes of paint finish the illusion.

Now, every time I reach for a pen, a small piece of deep time sits beside it, holding my papers down.

Both printed parts are reusable, so once you've made them you can cast as many fossils as you like.

Materials

1. 300 g cement
2. 150 ml water
3. About 54 g PLA filament (for both printed parts)
4. Release agent, such as petroleum jelly or cooking oil
5. Water-based paint in black, dark blue and white
6. Clear matte sealer (optional, but recommended)

Tools

1. 3D printer (I used a Bambu Lab printer with a textured PEI plate)
2. Mixing cup and stir stick
3. Small flat brush and a paint palette
4. Cloth or paper towel
5. Dust mask and gloves

Safety: don't breathe in cement dust, and keep wet cement off your skin because it is alkaline and irritating. Wear the mask while handling the dry powder and gloves while mixing.

Before making a fossil, it helps to know how nature makes one, because this project copies the same process.

1. The animal dies. An ammonite, a distant relative of today's squid and octopus that lived inside a coiled shell, dies and sinks to the seafloor.
2. It is buried quickly. Mud and sand cover the shell before scavengers, currents, or decay can destroy it. Quick burial is the key to becoming a fossil.
3. The soft parts disappear. The body rots away, but the hard shell stays pressed into the sediment.
4. The mud turns to stone. Over millions of years, more layers pile on top. Pressure and minerals carried by groundwater slowly harden the mud into rock.
5. The shell leaves its mark. Often the shell itself dissolves, leaving a hollow, shell-shaped imprint in the rock. This is called a mould fossil. If minerals later fill that hollow, the result is a cast fossil.
6. The fossil returns to the surface. Earth movements lift the rock, erosion wears it down, and eventually someone cracks it open.

How this project copies nature:

1. The seafloor mud is the wet cement.
2. The ammonite is the 3D-printed stamp.
3. Millions of years of hardening become a few days of cement curing.
4. The shell dissolving away is you lifting the stamp out.
5. The result is a mould fossil: a shell-shaped impression in solid stone.

Fun facts:

1. Ammonites evolved quickly and spread across the world's oceans. Because each species existed for a relatively short time, geologists use them as "index fossils" to date the rock layers they're found in.
2. In the Kali Gandaki river valley in Nepal, black ammonite fossils called Shaligrams are collected and worshipped as a sacred form of Vishnu. People have treasured these spirals for centuries.

The project needs two printed parts, and each has one job.

The ammonite stamp creates the fossil. It's a spiral shell with fine, wavy ribs, mounted on a flat circular disc. The disc works as a handle, so you can press the shell straight down with even pressure. The fine ribs are what make the finished fossil look real.

The hexagon mould gives the paperweight its shape. It's a shallow, open-top hexagonal tray that holds the wet cement while it sets. The finished paperweight is a solid cement hexagon, so the flat sides help it sit steady on a desk and make it easy to pick up. Make the tray deeper than the shell is tall, so the stamp can't push all the way through the cement.

The STL files are attached to this step so you can print your own.

Both parts fit on one build plate and print together.

1. Material: PLA
2. Filament used: 17.63 m (53.42 g)
3. Printing time: 1 h 38 min (1 h 46 min including preparation)

Print the stamp shell side up, with the disc flat on the plate. The ribs then print cleanly without supports. The fine layer lines on the shell will transfer into the cement, and once painted they look just like the growth lines on a real shell.

1. Put on your mask and gloves.
2. Weigh 300 g of cement into a cup.
3. Add 150 ml of water and stir for 1–2 minutes until the mix is smooth with no lumps. At a 2:1 ratio of cement to water, it pours easily and flows into fine detail.
4. Pour the mix into the hexagon mould until it reaches the rim.
5. Tap the mould on the table 20–30 times. Trapped air bubbles rise and pop, leaving a smooth surface.
6. Level the top with a flat stick.

You now have a flat, wet cement hexagon, ready for the fossil.

This is the heart of the project, and timing is everything.

1. Press too early, and the runny cement flows back into the impression and blurs the detail.
2. Press too late, and the cement cracks instead of taking the shape.

Wait until the surface has lost its wet shine and thickened, so a fingertip leaves a dent without cement sticking to it. Check every few minutes, because the right moment depends on how warm the room is.

1. Hold the stamp by its disc, shell facing down, centred over the cement.
2. Press straight down, slowly and evenly, until the shell is fully sunk into the surface.
3. Leave the stamp in place while the cement firms up.
4. When a fingernail barely marks the cement, lift the stamp straight up with a gentle wiggle.

As the shell comes out, the cement tears and lifts slightly around the edges of the impression. Leave those rough edges alone. They are what make the fossil look freshly dug out of rock.

Cement grows stronger when it dries slowly, just as the seafloor mud hardened slowly into rock.

1. Cover the mould loosely with plastic to keep the moisture in.
2. After about 24 hours, flex the mould gently and push the cement hexagon out from behind.
3. Let it keep curing for several more days before painting. If the room is hot and dry, mist it with water once a day.

Wash the stamp in warm, soapy water, and it's ready for the next fossil.

Bare cement looks plain. Paint turns it into ancient rock. The principle is simple: dark colour settles into the deep parts and the high points stay light, the way dirt and minerals collect in a real fossil.

1. Base wash. Thin dark blue or black paint with plenty of water and flood the fossil. The cement soaks it up and it runs into every groove.
2. Darken the spiral. While it's still damp, add stronger black to the centre of the spiral and between the ribs.
3. Marble the border. Dab a lighter mix of blue and white onto the flat area around the fossil in uneven patches, letting some bare cement show through. Real stone is never one flat colour.
4. Wipe the highlights. With a barely damp cloth, lightly wipe across the tops of the ribs. The paint lifts off the ridges but stays in the grooves, so the texture stands out.
5. Seal. Once it's completely dry, brush or spray on a clear matte sealer to protect the paint from hands and daily use.

I made two colour schemes:

1. Deep sea: a dark blue fossil in a pale, marbled blue stone
2. Slate: a charcoal fossil in smoky grey stone, like shale

Set it on your desk and let it hold down your papers.

No two paperweights come out the same. The edges break in their own way and the paint settles differently every time, so each one feels like a real find rather than a copy.

Once you have the stamp and mould, you can:

1. Make a matching set in different colour schemes
2. Give one to someone who loves fossils or the sea
3. Design new stamps, such as a trilobite, fern, or leaf, and use the same pressing method

This project started with a simple wish: to have a fossil on my desk. It turned into a small lesson in how the Earth keeps its memories. A shell presses into soft ground, the ground turns to stone, and the shape survives long after the shell is gone.

Making one yourself takes days instead of millions of years, but the feeling is the same when you lift the stamp and see the spiral staring back at you. It's a reminder that the things we shape carefully can last a very long time.

If you make one, post a photo with "I Made It!" I'd love to see the fossils you bring to life.