# Crabs’ eyes

**Lapides cancrorum*, *oculi cancrorum**

**Known today as.** Chalky stones from the stomachs of crayfish.

Kind: Animal

## What was it used for?

“Absorbents”: for acidity, heartburn, and fevers.

## What does modern science say?

They are calcium carbonate, a real antacid, so the remedy worked for heartburn. Calcium carbonate neutralises stomach acid, so the heartburn remedy was a sound one, the same as chalk and the antacid tablets of today; for fever it did nothing.

## Pharmacognosy and toxicology

- **calcium carbonate**: amorphous and calcite
- **calcium phosphate**: 
- **chitin**: the matrix

**Mechanism.** The gastroliths of the crayfish, stored calcium for its new shell after the moult: a chalk antacid, and nothing else. They were in the London Pharmacopoeia through the eighteenth century as an “absorbent” for acidity, which they are.

**Dose.** Inert.

**Standing.** Obsolete; the gastrolith is now studied for its amorphous calcium carbonate as a bioavailable calcium supplement.

## What lore surrounds it?

They are not eyes but gastroliths, flat buttons of lime; the name came from their look,. The *pulvis e chelis cancrorum*, of crabs’ claws, was the companion powder.

## What did the older authorities say?

**The German pharmacy.** *Lapides cancrorum* were gathered from the stomachs of crayfish in the rivers of Bavaria and Bohemia, where the animals store lime before moulting, and were sold by the pound to the apothecaries of Europe; the Dutch prepared them by levigation into a white powder.

**Culpeper.** Gives them, with coral and pearl, among the absorbent powders for the sour stomach, the heartburn, and the fevers of children.

## Sources

- Culpeper, N. (1653). *Pharmacopœia Londinensis*.
- Pomet, P. (1694). *Histoire générale des drogues*.
- Luquet, G. (2012). “Biomineralizations: insights and prospects from crustaceans.” *ZooKeys* 176.

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*Crabs’ eyes.* In *Materia Medica*, edited by B. C. P. Statema. Version 1, revised 2026-10-01. The Thinking Press. https://thethinkingpress.org/materia-medica/crabs-eyes

Historical description, not medical advice.