What Are Sharks Teeth Made Of? Composition & Structure

Shark teeth are made primarily of fluoroapatite, a calcium phosphate mineral with fluorine built directly into its crystal structure. This sets them apart from human teeth, which are based on hydroxyapatite, a related mineral where hydroxide ions sit where fluorine sits in the shark version. The difference is not trivial: fluorine makes shark teeth harder, more chemically stable, and remarkably resistant to the corrosive effects of seawater. Beyond this mineral foundation, shark teeth have a layered architecture and a nanoscale internal organization that researchers are still mapping in detail.

Fluoroapatite and the Enameloid Coating

The outermost layer of a shark tooth is called enameloid, and it is the hardest tissue in the tooth. Enameloid is sometimes compared to the enamel on human teeth, but the two differ in both chemistry and origin. Human enamel is made of hydroxyapatite and is produced by specialized cells called ameloblasts. Shark enameloid is made of fluoroapatite and develops through a process that involves both epithelial and mesenchymal cells, making it a hybrid tissue with no perfect analog in mammals.

The chemical formula for the mineral in shark enameloid is Ca₅(PO₄)₃F.¹ The fluoride content of shark enameloid is about 3.1% by weight, which is close to the fluoride content of pure geological fluoroapatite crystals.² That fluorine is not just sitting loosely in the mineral. Atomic-resolution imaging has shown that fluorine atoms form covalent bonds with surrounding calcium atoms within the crystal lattice, which directly strengthens the structure.³ This is also why fluoride is added to human toothpaste and drinking water: it partially converts hydroxyapatite to fluoroapatite, making tooth surfaces more resistant to acid. Sharks get this benefit natively, throughout the entire enameloid layer.

Fluorine is not evenly distributed across the tooth. Studies using laser-induced breakdown spectroscopy have found that fluorine is concentrated in the enameloid and essentially absent from the dentin underneath. Within the enameloid itself, the outermost “shiny layer” tends to have the highest fluorine concentration, and the tip of the tooth is richer in fluorine than the base.¹ This gradient makes intuitive sense: the tip and outer surface take the most abuse during biting, so concentrating the hardest mineral there puts protection where it matters most. Without fluorine bonded to calcium, the tooth becomes structurally weaker even though the phosphate-oxygen bonds in the mineral are themselves strong.

Inside the Enameloid at the Nanoscale

The mineral content alone does not explain why shark teeth perform so well mechanically. The internal architecture of the enameloid is what elevates it from a hard coating to a genuinely tough material. Under transmission electron microscopy, enameloid reveals a hierarchical arrangement of fluoroapatite crystallites that are extremely thin (roughly 50 to 80 nanometers wide) and very long (over 1,000 nanometers).⁴ These crystallites are not arranged randomly. They are densely packed and organized into bundles that run in specific directions depending on the region of the tooth.

In modern sharks, the majority of the enameloid interior is made up of what researchers call bundled crystallite enameloid, where the crystallites are grouped into parallel bundles that weave through the tooth in complex, three-dimensional patterns. The very outermost surface is covered in a thinner zone of single crystal enameloid, where crystallites are oriented perpendicular to the tooth surface.⁵ This layered arrangement is analogous in principle to plywood or woven composites: by stacking fibers in alternating orientations, the material resists cracking in any single direction. The result is that both the stiffness and hardness of the enameloid are surprisingly uniform regardless of which direction force is applied, even though the individual crystallites are highly directional.⁴

This kind of hierarchical organization at the nanoscale is one reason materials scientists have taken such an interest in shark teeth. A tooth that needs to puncture and saw through tough prey without chipping has engineering demands that overlap with those of cutting tools and armor. The natural solution sharks evolved, long thin crystallites woven into bundles within bundles, is a design principle that researchers are actively trying to mimic in synthetic materials.

The Dentin Underneath

Below the enameloid, the bulk of a shark tooth is made of dentin, a softer tissue that is chemically and structurally different from the enameloid above it. Dentin contains more water, more organic material (mainly collagen), and more carbonate than enameloid, but less fluoride.² It is still a mineralized tissue, but it behaves more like bone in some respects and is considerably softer than the enameloid shell.

Not all shark species build their dentin the same way, and researchers have identified three distinct tooth types, or histotypes, based on how the interior is organized:

• Orthodont teeth: These have a central hollow pulp cavity surrounded by a thick layer of compact dentin called orthodentine, which has tightly packed parallel tubules running through it. Many requiem sharks and hammerheads have this type.
• Osteodont teeth: These lack orthodox dentin entirely. Instead, the entire tooth crown is filled with osteodentine, a porous dentin that looks a lot like spongy bone, with vascular canals surrounded by concentric layers of tissue resembling osteons. Great white sharks and makos have osteodont teeth.
• Pseudoosteodont teeth: These are a middle ground, where the pulp cavity starts out hollow during development but is later filled in by osteodentine, while still retaining an outer shell of orthodentine.

These three histotypes were first described decades ago and remain a useful way to categorize shark teeth.⁶ The osteodentine filling in osteodont and pseudoosteodont teeth is particularly interesting because it contains a complex three-dimensional network of canals. Primary canals range from about 10 to 120 micrometers in diameter, and from those, hierarchically branching secondary canals extend outward, shrinking down to just hundreds of nanometers across.⁷ This vascular network likely plays a role in controlling how the dentin mineralizes during tooth development, compensating for the fact that osteodentine lacks the living bone cells that normally regulate mineralization in mammalian bone.

How Hard Are Shark Teeth Compared to Human Teeth

Given that shark enameloid is loaded with fluoroapatite while human enamel is hydroxyapatite, you might expect shark teeth to be dramatically harder. The reality is more nuanced. Nanoindentation testing on shark teeth from multiple species has found that shark enameloid and human enamel are comparable in hardness.² The same is true for their respective dentin layers. So a shark tooth is not some super-hard exotic material compared to yours; it is roughly in the same league, at least in terms of raw hardness numbers.

Where shark teeth do stand out is in the internal contrast between layers. Shark enameloid is roughly six times harder than the dentin beneath it.² This steep gradient matters functionally. A very hard outer shell resists wear and penetration during biting, while a softer, more flexible interior absorbs stress and reduces the chance of catastrophic fracture. A tooth that was uniformly as hard as enameloid would be brittle. A tooth that was uniformly as soft as dentin would dull quickly. The combination is what makes the tooth work.

Mechanical testing has also shown that the stiffness (Young’s modulus) and hardness of enameloid are consistent across different shark species, even when those species have different tooth shapes and diets. In nanoindentation comparisons between a bonnethead shark (which eats crustaceans and seagrass) and a sand tiger shark (which eats fish and squid), there was no significant difference in enameloid properties.⁸ This suggests that the enameloid recipe is fairly conserved across sharks, with differences in tooth performance coming more from shape, dentin type, and overall structure than from changes in the enameloid mineral itself.

Continuous Replacement and How New Teeth Mineralize

Sharks do not get one set of teeth and hope for the best. Most species continuously produce new teeth throughout their lives, cycling through thousands over a lifetime. This is managed by a structure called the dental lamina, a strip of tissue that runs along the jaw and acts like a conveyor belt. New teeth form at the back of the jaw and migrate forward to replace older teeth as they are lost or shed.

The dental lamina in sharks is a highly proliferative unit, constantly growing and constantly generating new tooth buds. Research on the catshark has found evidence that stem cells reside at the tongue-facing edge of the dental lamina. These cells divide slowly, feeding daughter cells into the faster-dividing population that builds each new tooth. A pulse-chase experiment tracked cell proliferation and found that the resulting population shifted one tooth position toward the mouth surface over about four to five weeks.⁹ The speed of replacement varies by species, water temperature, and other factors, but the underlying mechanism is the same: a permanent reservoir of stem cells that never runs out.

This regenerative capacity has attracted attention from biomedical researchers. The stem cell markers expressed in the active shark dental lamina turn out to overlap with markers found in resting dental tissue in humans, tissue that still exists in adult human jaws but is normally dormant.¹⁰ This does not mean we are close to regrowing human teeth, but it does mean the genetic machinery for continuous tooth production is not unique to sharks. Humans may retain vestiges of it, and understanding the shark version could eventually inform regenerative dentistry.

Each new tooth goes through its own mineralization process. The enameloid starts as a protein-rich matrix that is gradually replaced by fluoroapatite crystallites, which lengthen and organize themselves into the bundled pattern described earlier. The dentin beneath mineralizes separately, using collagen as a scaffold. By the time a tooth reaches the functional position at the front of the jaw, it is fully mineralized and ready to bite.

Resilience to Ocean Acidification

One practical consequence of being made from fluoroapatite rather than hydroxyapatite is chemical stability in seawater. As ocean pH drops due to rising carbon dioxide levels, many calcified marine structures, like coral skeletons and the shells of some mollusks, dissolve more readily. Shark teeth appear to resist this effect. In experimental exposures to acidified seawater, shark teeth remained structurally intact and were primarily composed of fluoroapatite. In fact, the teeth exposed to lower pH conditions showed an increase in fluoride content and higher crystallinity, suggesting that sharks can actually enhance their tooth mineralization in response to acidification to produce teeth that are more resistant to corrosion.¹¹

Fluoroapatite is one of the least soluble calcium phosphate minerals, and its low solubility is directly tied to the fluorine in the crystal lattice. This property is not just good for living sharks; it is also why shark teeth dominate the marine fossil record.

Why Shark Teeth Are So Common as Fossils

If you have ever found a fossil on a beach, there is a good chance it was a shark tooth. This is partly because sharks produce so many teeth over their lifetimes, but it is also because of what those teeth are made of. The fluoroapatite in enameloid is already close to its most thermodynamically stable form, so it resists the chemical changes that break down most biological hard tissues over geological time. Fossil shark teeth are actually denser than fresh ones, because over millions of years, additional fluoride diffuses into the tooth from the surrounding sediment, further stabilizing the crystal structure.¹²

In living shark teeth, the enameloid contains fluoroapatite while also harboring some amorphous calcium phosphate and defect apatite with lower calcium-to-phosphorus ratios. During fossilization, these less stable phases convert to more crystalline fluoroapatite, and fluorine from groundwater fills any remaining hydroxide positions in the crystal lattice. The result is a fossil that is chemically purer and harder than the original tooth. This is also why fossil shark teeth come in those distinctive dark browns and blacks: the color comes from minerals like iron and manganese that seep into the porous dentin over time, while the enameloid stays relatively unchanged.

An interesting difference between modern and fossil shark teeth is where the fluorine sits. In living sharks, fluorine is found almost exclusively in the enameloid. In fossil teeth, fluorine has migrated into the dentin as well.¹ Paleontologists use this kind of information to assess how well-preserved a fossil tooth is: if the original chemical gradients are still intact, the tooth’s isotopic signatures can be trusted for reconstructing ancient ocean temperatures and environments. If fluorine has penetrated everywhere and the original composition is homogenized, the geochemical data may be compromised.¹³

Teeth, Skin, and a Shared Developmental Blueprint

Shark skin feels like sandpaper, and this is not a coincidence of texture. The tiny structures covering shark skin, called dermal denticles, are structurally and developmentally related to teeth. Each denticle has a crown of enameloid over a core of dentin, with a pulp cavity inside, just like a miniature tooth. The connection runs deeper than appearance: gene expression studies have found that shark denticle development uses a vast set of the same genes as tooth development. This shared genetic signature suggests that teeth and denticles descend from a common ancestral structure, an ancient epidermal appendage that predates the split between jawed and jawless vertebrates.¹⁴

The teeth-from-scales hypothesis has been debated for over a century, and the molecular data now strongly supports the idea that teeth did not evolve independently in the mouth but were co-opted from skin structures that already had the machinery to produce hard, mineralized points. In sharks, you can still see the continuum: denticles on the skin transition into larger, more complex teeth at the jaw margin, and in some species the boundary between “skin” and “mouth” denticles is blurry.

This shared origin also connects to the broader question of how vertebrate mineralized tissues evolved. Studies comparing enamel and enameloid formation across species, from sharks to frogs to mammals, have found that the basic process of organizing long, parallel mineral crystals into a hard surface layer is ancient and widespread.¹⁵ Sharks, with their fluoroapatite enameloid and continuous tooth replacement, represent one of the oldest and most successful variations on this theme. The toolkit they use to build teeth has been running, largely unchanged, for hundreds of millions of years.