What Are Eggshells Made Of?

An eggshell looks simple from the outside. It is not. A chicken eggshell is a thin, layered mineral structure — mostly calcium carbonate — built on a protein scaffold that a hen produces in a single day. The shell has to be strong enough to protect a developing chick, yet porous enough to let oxygen in and carbon dioxide out. It is one of the more remarkable materials in nature, and it is also the subject of a lot of questionable health marketing.

By weight, a chicken eggshell is roughly 95% calcium carbonate, with the rest made up of proteins, water, and small amounts of other minerals. The calcium carbonate is what gives the shell its hardness. The protein framework is what keeps it from being brittle like chalk. Understanding what the shell is made of explains both why people grind it into powder and why that powder is not the simple calcium supplement some sellers claim.

What Are Eggshells Made Of?

A chicken eggshell is a calcium carbonate mineral structure held together by a protein matrix. Calcium carbonate makes up about 95% of the shell by weight. The remaining few percent includes proteins such as ovocleidin and osteopontin, plus trace amounts of magnesium, phosphorus, and other minerals.

The shell is not one solid layer. It has an inner and outer membrane, a mammillary layer where crystals first form, a palisade layer of densely packed crystal columns, and a thin outer cuticle. Each layer forms at a different stage as the egg travels through the hen’s oviduct.

Here is the part that surprises most people. The shell is not deposited as a finished solid. It grows crystal by crystal on a protein scaffold over roughly a day. The hen supplies the calcium from her own skeleton and diet. That is why a hen’s calcium intake matters so much for shell quality.

Why Is Calcium Carbonate the Main Ingredient?

Calcium carbonate is the same mineral found in limestone, chalk, and marble. In the eggshell it is arranged in a crystal form called calcite, and that arrangement is what makes the shell strong in the thin layer the hen can afford to build.

The chemistry is straightforward. Calcium ions combine with carbonate ions to form calcium carbonate. The hen’s body pulls calcium from her bloodstream and from reserves in her bones, then deposits it onto the protein framework in the shell gland. The carbonate comes from the hen’s normal metabolism.

What makes the shell work is not just the mineral. It is the way the mineral is organized. Pure calcium carbonate would be brittle. The protein scaffold controls how the crystals grow, giving the shell a structure that resists cracking while staying thin enough for the embryo to breathe. This is why shell strength is about architecture as much as raw calcium.

What Is the Protein Part of an Eggshell?

The protein in an eggshell is a small fraction of its weight but does most of the structural work. Proteins like ovocleidin-17 and osteopontin help control where and how the calcium carbonate crystals form. Without them, the shell would not assemble correctly.

These proteins are not just filler. Research has found that they influence crystal formation and may contribute to the shell’s mechanical properties. The exact roles are still being studied. What is clear is that the shell is a composite material — mineral plus protein — and the two parts work together.

The shell membranes, the thin layers just inside the hard shell, are mostly protein fibers. They act as a barrier against bacteria and as a scaffold for the mineral layer. When people talk about eggshell powder, they usually mean the whole shell ground up, membranes included.

Does Eggshell Powder Work as a Calcium Supplement?

Eggshell powder does contain calcium in a form the body can absorb. That part is not in dispute. Calcium carbonate is a standard calcium source, and the calcium in eggshell powder is largely calcium carbonate. The question is whether the powder is a good or necessary way to get it.

The evidence is limited. Some small studies have looked at eggshell calcium and found it can raise calcium intake and may be absorbed, but the research is not large or strong enough to call it a proven superior supplement. It has not been shown to be better than standard calcium carbonate or calcium citrate.

There are practical concerns too. Eggshells can carry bacteria such as Salmonella on their surface, and home grinding does not reliably sterilize them. Raw or lightly treated eggshell powder carries a real food safety risk. Commercial products that heat-treat the shells reduce that risk, but the safety of any given product depends on how it was processed.

No clinical guidelines recommend eggshell powder over standard calcium supplements. If you need more calcium, food sources and standard supplements have far more evidence behind them. Anyone considering eggshell powder should talk to a clinician first, especially people with kidney problems or a history of calcium-related conditions.

Is Eggshell Calcium Better Than Other Calcium Sources?

No good evidence shows eggshell calcium is better than other calcium sources. It is essentially calcium carbonate, which is the same compound used in many common supplements and found in foods. There is no established advantage in absorption or effect.

Calcium carbonate is absorbed best when taken with food, because it needs stomach acid to dissolve. Calcium citrate is often easier on the stomach and does not require food. Those are well-established differences between supplement forms. Eggshell powder behaves like calcium carbonate in this respect.

What matters for most people is total calcium intake, not the specific source. Dairy foods, fortified foods, leafy greens, and standard supplements all contribute. Eggshell powder is one option among many, and not a well-studied one.

What Else Is in an Eggshell Besides Calcium?

Eggshells contain small amounts of other minerals, including magnesium, phosphorus, and trace elements. The shell membranes add protein. The amounts are small compared with the calcium carbonate, so the shell is not a meaningful source of most other nutrients.

Some products market eggshell powder as a source of collagen or other compounds from the membrane. The evidence for these claims is thin. The membrane does contain proteins, but no strong human trials show that eating ground eggshell delivers them in a useful way.

One clarification worth making: the color of an eggshell — white or brown — has nothing to do with its mineral content. Shell color comes from pigments deposited by the hen and is determined by breed. A brown shell and a white shell are made of the same calcium carbonate structure.

Can You Eat Eggshells Safely?

Eating eggshells is not automatically safe. The main risk is bacterial contamination, particularly Salmonella, which can be present on shell surfaces. Grinding shells at home does not reliably kill bacteria.

Commercial eggshell calcium products typically use heat treatment to reduce this risk. That processing is what separates a regulated product from a home-made powder. If someone chooses to consume eggshells, using a properly processed product is safer than grinding raw shells in a kitchen.

There are also people who should be cautious with any extra calcium. People with kidney disease, a history of kidney stones, or certain parathyroid conditions may need to limit calcium. Calcium supplements can interact with some medications, including certain antibiotics and thyroid medicines. A clinician or pharmacist can advise on timing and safety.

Why Does the Shell Structure Matter?

The shell’s layered structure is what lets an egg do two opposite jobs at once. It protects the contents from the outside world while allowing gas exchange for the embryo. A solid, impermeable shell would suffocate the chick. A weak shell would let in bacteria and break too easily.

The pores in the shell allow oxygen and carbon dioxide to move through. The cuticle on the outside helps limit water loss and blocks some microbes. The membranes inside add another barrier. These features are why the egg is such an effective package for a developing bird.

For people, the takeaway is that the shell is a designed composite, not a simple chunk of chalk. That design is why it is strong, and it is also why grinding it up does not turn it into a special health product. It is still mostly calcium carbonate.