carbonates
Aragonite
Ca(CO3)
- Also known as
- sputnik aragoniteflos ferri
- Crystal system
- orthorhombic
- Hardness
- 3.5–4 on the Mohs scale
- Lustre
- vitreous
- Colour
- White, colourless, yellow, blue
- Where it forms
- From warm or agitated water rich in calcium, and biologically. Over geological time it converts to calcite, which is why very old aragonite is rare.
- Chiefly from
- Molina de Aragon, SpainMoroccoSicilythe ocean
Why it is that colour
Impurities. Pure aragonite is colourless.
Safety
Safe to handle; dissolves in acid as calcite does.
Where it comes from
Common. Moroccan and Spanish specimen material is widely traded and inexpensive.
Historical use
Biologically the more important of the two calcium carbonates. Mother of pearl is aragonite; so is pearl itself, and so are the skeletons of reef-building corals. Ocean acidification is a threat specifically because aragonite dissolves more readily than calcite as seawater pH falls.
Recorded as history and attributed where possible. That a stone was used for something establishes that people used it, not that it worked.
What is attributed to it, and since when
No traditional attribution. Aragonite was distinguished from calcite in 1797 and named after the Spanish locality. Its meanings are modern.
Described, not endorsed. Dating a claim is not the same as assessing it — but an association that first appears in a book published in 1985 is not an ancient inheritance, whatever the label says. A sincere account of feeling better is compatible with the stone doing nothing — see the placebo effect.
Worth knowing
Aragonite and calcite have the same formula and are different minerals. They differ only in how the atoms are packed, and that difference is enough to change the crystal system, the hardness, the density and the stability. Two substances, chemically identical, genuinely distinct — which is what mineralogists mean by polymorphs, and it is the same relationship as diamond and graphite.
The ocean chemistry matters. Corals and many shellfish build with aragonite, which is the less stable form, and as the sea absorbs carbon dioxide and grows more acidic the saturation state for aragonite falls first. The reason coral reefs are named as an early casualty of ocean acidification is a fact about this mineral.
Identity checked against IMA-CNMNC master list, June 2025.