Gem library
Pearl
The only gem an animal makes whole
The pearl is the only gem harvested ready to wear: no cutting, no polishing. A mollusc builds it layer by layer, and it is that stack of aragonite platelets that gives it its orient, the shifting sheen no imitation truly renders.
- Hardness (Mohs)
- 2.5 – 4.5
- Density
- 2.60 – 2.85
- Crystal system
- Orthorhombic
- Refractive index
- 1.52 – 1.69
- Cleavage
- None
Mineralogical identity
- Family
- Organic gem (biomineral)
- Species
- Nacre
- Variety
- Freshwater cultured, Akoya, Tahitian, South Sea
- Chemical formula
- CaCO₃ (aragonite 82 – 86 %) + conchiolin
- Crystal system
- Orthorhombic (aragonite in platelets)
- Classification
- Organic gem
- Rarity
- Common
- Indicative price
- €1 – 500/piece (natural fished pearls: several thousand)
Etymology
From the Latin perna, «ham», for the shape of the shell that holds it, by way of the vulgar form perula.
Physical & optical properties
- Hardness (Mohs)
- 2.5 – 4.5
- Density
- 2.60 – 2.85
- Refractive index
- 1.52 – 1.69
- Birefringence
- 0.156
- Cleavage
- None
- Fracture
- Splintery
- Lustre
- Pearly
- Diaphaneity
- Opaque
- UV fluorescence
- Weak
- Pleochroism
- None
- Colours
- white, cream, pink, grey, black, golden
Formation, geology & history
How a mollusc makes a stone
The mantle of the mollusc secretes aragonite, a form of calcium carbonate, in hexagonal platelets half a micron thick, cemented by a protein, conchiolin. Around a foreign body, these layers stack by the thousand. The orient, that characteristic iridescent shimmer, arises from the interference of light between the platelets: the thickness of each is of the same order as the wavelength of visible light, and the pearl behaves like a stack of thin films. The finer and more regular the layers, the deeper the orient. It is exactly the mechanism that colours a soap bubble, except that here it is frozen in matter.
Natural, cultured, imitation
A natural pearl forms without human intervention, by accident: they are exceedingly rare and come mainly from the historic Persian Gulf fisheries. A cultured pearl results from a graft introduced by a human, and accounts today for almost the whole market. It is not an imitation: the pearl is genuinely secreted by the animal, and the distinction concerns what triggered it, not the material. Radiography settles it, by showing the presence or absence of a nucleus. An imitation, by contrast, is a bead of glass or plastic coated in essence d'orient, and that is no longer a pearl at all. The invention of culture by Kokichi Mikimoto, patented in 1896, collapsed the natural pearl market within one generation.
The great families
Freshwater pearls, chiefly Chinese, are the most plentiful and the cheapest: often without a nucleus, hence entirely nacre. Japanese Akoya, round and highly lustrous, are the classic pearls of the white necklace. Tahitian pearls, grey to black, come from the black-lipped oyster of Polynesia. South Sea pearls, white or golden, from Australia and Indonesia, are the largest and the costliest. Price depends on five criteria: lustre, surface, shape, colour and size, with lustre outweighing all the rest.
How to recognise it
- The tooth testRub the pearl gently along the edge of an incisor: nacre grips, it grates slightly, it is rough at the microscopic scale. An imitation slides, smooth as glass. The test is old and remains the best.
- A loupe on the drill holeOn a real pearl the nacre thins around the hole and shows concentric layers. On an imitation the coating flakes and reveals the supporting bead.
- IrregularitiesNo pearl is perfect: you see folds, small hollows, differences of tint. A strand of rigorously identical pearls is not natural.
Imitations, treatments & confusions
- Glass pearls with essence d'orientMajorica pearls and their equivalents: a glass bead coated in a varnish based on fish scales. Smooth to the tooth, too regular, and heavy.
- Plastic pearlsToo light for their size, warm to the touch, and the coating flakes at the drill hole.
- Undeclared dyed pearlsDyeing and irradiation are common, particularly for greys and blacks. It is not fraud if it is declared. An unusual colour at a low price should prompt the question.
Deposits & origins
China (most freshwater pearls), Japan (Akoya), Australia and Indonesia (South Sea), Polynesia (Tahiti), Bahrain and the Persian Gulf (historic fisheries)
Care & precautions
- Never prolonged water, never a product: perfume, hairspray and vinegar attack nacre, which is a carbonate.
- Put them on last when dressing, take them off first. Cream and perfume are their worst enemies.
- Wipe with a soft cloth after each wearing, perspiration is acidic.
- Store flat in a cloth, never in an airtight bag: nacre needs a little ambient humidity and dries out in a safe.
Traditions & crystal healing
- Chakras
- Heart · throat
- Zodiac signs
- Cancer · Pisces
- Element
- Water
- Month
- June
Associated since antiquity with purity, femininity and the stages of life, it is the stone of June and the traditional gem of the thirtieth wedding anniversary. The Romans saw in it the tears of the moon. Cultural heritage, never medicine.
The virtues described here belong to tradition and cultural heritage. They are in no way medical advice and never replace a consultation.
Frequently asked questions
Is a cultured pearl a real pearl?
Yes. The nacre is secreted by the animal, exactly as in a natural pearl. The difference lies in the trigger, induced rather than accidental, and it must be stated at sale.
Why have my pearls yellowed?
The nacre dries out and the conchiolin oxidises, often after years in a safe or in contact with perfume. The process is not reversible: wear them, that is what preserves them best.
Can pearls be cleaned with soapy water?
A damp cloth, yes, briefly. Soaking, no: water penetrates the thread of the necklace, which rots and breaks, and it dulls the nacre.
Tools for this stone
Pearl: only the tools this stone actually concerns, each carrying the value read on this page.