What Pearls Are Made Of

Most gem pearls are made of nacre, a composite of tiny aragonite crystals (a form of calcium carbonate) stacked in thin layers and glued together by an organic protein called conchiolin, plus a little water. The same material lines the inside of the shell as mother-of-pearl. The way those layers are arranged produces a pearl's luster and soft sheen. It also explains why pearls are soft and sensitive to acid.

The building blocks

Calcium carbonate (CaCO3) makes up the large majority of a pearl's weight, often quoted at around 80 to 90 percent or more for nacreous pearls. It crystallizes as aragonite, the orthorhombic form of calcium carbonate, rather than as calcite, the trigonal form found in limestone and in the outer layers of many shells. The rest is conchiolin and water, along with trace elements picked up from the water the mollusk lived in.

Conchiolin is a tough mix of proteins and the sugar polymer chitin. Brown and fairly flexible, it forms thin sheets between the mineral layers and walls between individual crystals. It gives nacre much of its toughness, and it also degrades slowly with age, heat and dryness.

Trace elements matter more than their tiny amounts suggest. Freshwater pearls contain more manganese than saltwater pearls, which labs use to separate the two. Saltwater pearls carry more strontium. The page on gemological instruments describes the X-ray and chemical tools that read these differences.

How nacre is built

Under an electron microscope, nacre looks like a brick wall. Each brick is a flat aragonite tablet, usually a few micrometers across and roughly half a micrometer thick, about the same thickness as a wavelength of visible light. The mortar is a very thin film of conchiolin. Thousands of these layers wrap around the pearl's center like the layers of an onion.

The mantle cells of the pearl sac lay down nacre continuously while the mollusk is alive and the water is warm enough. Growth slows in cold water. Akoya farmers in Japan traditionally harvested in winter for this reason, since slow growth in cool water gives tighter, smoother layers and a brighter surface.

This structure is part of why nacre is much tougher than pure aragonite. A crack has to wind around each tablet through the organic layers instead of running straight through. Materials scientists have studied nacre for decades as a model for tough composites.

Luster and orient

Luster is the quality of light reflected from a pearl's surface and from just beneath it. On a fine pearl it looks sharp and bright, with a clear reflection of nearby objects. It comes from light bouncing off many smooth, regular nacre layers. Thin, uneven or poorly aligned layers scatter light and give a chalky, dull look.

Orient is the soft play of rainbow colors that seems to move across the surface of some high-quality pearls. It forms through light interference and diffraction. Light reflecting from successive thin layers overlaps and reinforces some wavelengths while cancelling others, and the edges of the stacked tablets act like a fine diffraction grating. The gemstone optics page covers iridescence and related effects in other gems.

Overtone is a related idea. It is a second color, such as rose, green or silver, that floats over a pearl's main body color. Overtone and orient both rely on the fine layered structure, and graders assess them together with body color.

Where pearl color comes from

A pearl's body color depends on the species of mollusk, the color of the nacre made by the donor tissue in cultured pearls, and pigments within the conchiolin. Organic pigments help produce the dark colors of Tahitian pearls and the gold of some South Sea pearls. Many freshwater pearls owe their pinks and purples to pigments too. The broader topic of what colors gems is covered on the color in gemstones page.

Some pearls also show a layer of dark conchiolin or a dark center that shows through thin nacre. This can give a bead-nucleated pearl a gray or blue cast.

Non-nacreous pearls

Not all pearls have nacre. Some mollusks build pearls from other arrangements of calcium carbonate that lack the brick-wall layering, so the surface looks like porcelain instead of showing orient. Gemologists call them non-nacreous pearls or calcareous concretions. They are still pearls in the trade sense when they form inside a mollusk.

• Conch pearls grow in the queen conch, a large Caribbean sea snail. They are pink to orange and often show a shimmering flame structure caused by fibrous bundles of aragonite. Their color can fade with long exposure to strong light. See the conch pearl page.
• Melo pearls come from the melo melo sea snail of Southeast Asia. They are usually orange, can grow large and also show flame structure.
• Quahog pearls come from a North American Atlantic clam and are often purple to white.
• Scallop and giant clam pearls also occur and are generally non-nacreous.

None of these are cultured commercially, so nearly all of them are natural. The organic gems section places them alongside amber, coral and other gems from living things.

Physical properties

The layered, organic-rich structure gives pearls a set of properties quite unlike those of most gem minerals.

On the Mohs hardness scale, pearls sit near calcite and fluorite. Quartz dust and harder gems can scratch them, which is why pearls should be stored apart. The specific gravity values help separate pearls from many imitations, which are often lighter plastic or heavier glass.

Why pearls need special care

Calcium carbonate reacts with acid. Vinegar, citrus juice, sweat with low pH and some cleaning products can etch nacre and dull its luster. The famous legend of Cleopatra dissolving a pearl in vinegar plays on this reaction, though a large pearl would dissolve very slowly in a drink.

Conchiolin can dry out and crack in very dry conditions or long storage in a safe, and heat speeds the process. Pearls kept for decades in dry, closed spaces sometimes develop fine cracks or peeling nacre. Wearing pearls lets them pick up a little moisture from the air and skin. For safe methods, see how to clean jewelry safely.

Frequently asked questions

Are pearls a mineral?

Pearls contain the mineral aragonite, but they are classed as organic gems because a living animal produces them and they contain organic material. They do not meet the strict definition of a mineral, which requires an inorganic origin.

Why do some pearls look chalky?

A chalky look usually means thin or poorly formed nacre, surface damage from acid or cosmetics, or a non-nacreous structure. Smooth, well-stacked layers are needed for bright luster.

Is mother-of-pearl the same material as a pearl?

Yes, mother-of-pearl is the nacre that lines the inside of a mollusk's shell. A nacreous pearl is made of the same material laid down in concentric layers around a center.