The tooth test is simple: rub the pearl gently along the edge of a front tooth. A real pearl feels faintly gritty or sandy; an imitation feels smooth. The test has been used by pearl dealers for generations, and unlike most folk tests it has a real physical basis. It also has a very narrow scope, and buyers regularly ask it to answer questions it cannot.
Why it works
Nacre is built of microscopic platelets of aragonite laid down in overlapping layers, like tiles on a roof. On the surface of a pearl these platelets form a fine, scaly texture — visible at 10× as the "fingerprint" pattern. Against the sensitive edge of a tooth (or, less romantically, rubbed against another pearl), the texture registers as grittiness.
A coated imitation — glass, plastic or shell under lacquer — has a smooth surface. No platelets, no grit.
What it catches
Coated imitation pearls with smooth lacquer. That is the whole list. For that purpose it is reasonably reliable, and it takes two seconds.
What it misses
Natural versus cultured. Both are nacre. Both are gritty. The tooth test cannot distinguish them; nothing on the surface can. That question is decided by X-ray. X-ray of pearls.
Treatments. Bleached, dyed, irradiated and coated cultured pearls are all nacre and all gritty (a lacquer coating over a real pearl may reduce grit, but not reliably).
Nacre thickness. A cultured pearl with a paper-thin layer of nacre over a shell bead is gritty. It will also wear through in a year.
Shell beads. Uncoated mother-of-pearl beads are nacre and feel gritty; they are not pearls.
Textured imitations. Some imitations are given a fine surface texture to defeat the test. They may feel slightly gritty; the drill hole gives them away.
Quality of any kind.
The better home check
Look at the drill hole with a 10× loupe. On a coated bead, the coating shows as a distinct layer at the rim, often chipped, with a different material beneath. On a real pearl the nacre continues cleanly into the hole. This is more reliable than the tooth test and does not involve putting the pearl in your mouth. Imitation pearls.
What the laboratory does instead
Surface examination at magnification for platelet structure; drill-hole examination; weight and SG; UV fluorescence; and X-ray for nucleus and nacre thickness. The report states natural or cultured (and type), treatments observed, and — where relevant — nacre thickness. Pearl (Moti): natural, cultured and imitation.
Frequently asked questions
My pearls feel gritty. Are they real?
They are nacre — genuine cultured pearls, natural pearls or shell beads. Which of those, the tooth test cannot say.
My pearls feel smooth. Are they fake?
Probably coated imitations, though a lacquered real pearl or a very fine, tightly layered pearl can feel smoother than usual. Check the drill hole.
Can the tooth test damage a pearl?
Gentle rubbing does not. Pearls are soft (2.5–4.5); biting or scraping can mark them.
Is there a better quick test?
The drill hole under a loupe, and a comparison of weight against size. Neither replaces a laboratory for anything that matters.
Why is the tooth test so famous?
Because it is easy, memorable and works for the one thing it works for. Its fame has outgrown its usefulness.