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Aragonite vs Calcite: Telling the Two Calcium Carbonates Apart

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: chemistry will not separate them, because both are CaCO3 and both fizz in acid. Structure does. Calcite is trigonal, hardness 3, specific gravity about 2.71, and splits cleanly into rhombohedral blocks. Aragonite is orthorhombic, hardness 3.5 to 4, specific gravity about 2.93, breaks unevenly, and grows as needles, sprays and six-sided twinned prisms. Look at the shape, then at a broken edge; a copper coin and a kitchen scale confirm it.

Why are two minerals sold under one formula?

Because the calcium and carbonate ions can stack in two ways. Minerals.net puts it plainly: calcium carbonate forms as both aragonite and calcite, and the two only differ in their crystallization. Mineralogists call such pairs polymorphs. A third arrangement, vaterite, is rare enough that collectors almost never meet it.

The trouble for buyers is that the two overlap in color, luster and price, and the same deposit can hold both. Minerals.net warns that when crystals are too small to see, only optical or x-ray work can tell the species apart, and that microcrystalline material may be labeled as the wrong mineral. Banded "onyx", cave growths and many tumbled stones fall in that gray zone. Coarse crystals, on the other hand, are easy, and the tests below are aimed at them.

Which one is in your hand?

Work through these in order. The first two settle most specimens.

Shape

Calcite's commonest forms, per Minerals.net's calcite page, are rhombohedrons and scalenohedrons: slanted boxes and sharp, double-pointed "dogtooth" crystals. Aragonite favors long prisms, needles, radiating sprays and columns. Its big crystals are usually trillings, three crystals twinned together into a six-sided column, so a hexagonal outline on a carbonate points to aragonite, not calcite. Under a loupe, a trilling's flat end shows grooves running in several directions, one set from each twin member.

The break

This is the decisive one. Calcite has perfect cleavage in three directions; a chip comes away as a little rhomb with flat, shiny faces, and cracks inside a clear piece run as straight, glinting planes. Aragonite has only one distinct cleavage and otherwise fractures in curved, subconchoidal surfaces. Minerals.net's distinguishing note for the pair is exactly this: calcite shows perfect rhombohedral cleavage and is slightly softer.

A copper coin

Webmineral places aragonite at 3.5 to 4, between a copper penny and fluorite. Geology.com's hardness chart gives copper sheet a hardness of 3 and calcite exactly 3. With firm pressure on an unpolished spot, copper will mark calcite or leave a smear on it, while aragonite shrugs the coin off. Both are scratched easily by a steel knife.

Weight in water

Aragonite is about 8 percent denser. Webmineral lists 2.93 for aragonite and 2.71 for calcite. Hang a clean, solid piece from a kitchen scale by a thread, weigh it in air and then submerged in water, and divide the dry weight by the loss. A 100 gram calcite loses about 36.9 grams in water; a 100 gram aragonite loses about 34.1 grams. Porous or matrix-heavy pieces give muddled results, so use a solid crystal. The specific gravity guide gives the full method.

What does not work

Acid. Geology.com's acid test table lists both aragonite and calcite as reacting strongly to cold and warm hydrochloric acid, so the fizz confirms a carbonate but not which one. Birefringence does not help either: Webmineral gives calcite 0.154 to 0.174 and aragonite 0.156, so both double a printed line through a clear piece. UV is suggestive at best. Webmineral lists calcite's colors as yellow, blue, red and green, and aragonite's as green and yellowish white in shortwave and green, yellow and pink in longwave, with wide overlap between them.

Side by side

Property Calcite Aragonite
Formula CaCO3 CaCO3
Crystal system Trigonal Orthorhombic
Mohs hardness 3 3.5 to 4
Specific gravity 2.71 2.93
Cleavage Perfect, three directions (rhombohedral) One distinct direction; subconchoidal fracture
Common habits Rhombs, dogtooth scalenohedrons, cave growths Needles, sprays, six-sided twinned prisms, coral-like flos ferri
Optics Uniaxial negative Biaxial negative
Reaction to cold dilute acid Strong Strong
Stability at the surface Stable Slowly converts to calcite

Data from Webmineral's calcite and aragonite pages and Minerals.net.

Where does each one form?

Calcite is the default. Geology.com calls it a rock-forming mineral found in sedimentary, metamorphic and igneous rocks alike, and the main constituent of limestone and marble (Geology.com, calcite). Most cave formations, veins and geode linings are calcite.

Aragonite needs particular conditions. Minerals.net's calcite page notes that among the calcium carbonate growths that build up from mineral-rich water, aragonite generally forms only at hot springs, and most others are calcite. It also says travertine, the porous mound rock built by hot springs, is usually aragonite. Aragonite is also the mineral of pearl and mother-of-pearl, and of coral skeletons. Left alone at the surface, it slowly reverts: the International Gem Society notes that heating to 400 °C converts aragonite into calcite, a change that also happens naturally over millions of years. Our pseudomorphs guide explains what happens when the outward shape survives that change.

The two carbonates in the ocean

The difference matters far beyond collecting. NOAA reports that in the 200-plus years since the industrial revolution began, surface ocean pH has fallen by 0.1 units, roughly a 30 percent increase in acidity (NOAA, ocean acidification). Which carbonate an animal builds with decides how exposed it is. The Smithsonian Ocean portal explains that corals use aragonite, while urchins, starfish and most coralline algae build from high-magnesium calcite, which dissolves even more quickly than aragonite or ordinary calcite.

Which trade names mix them up?

Several common shop names cross the line between the two species, which is where most mislabels start.

  • Blue aragonite and Caribbean calcite. Bright blue tumbles sold as blue aragonite are not always aragonite. Our calcite guide describes Caribbean calcite, a Pakistan and Afghanistan border material that is also marketed as blue aragonite and combines blue calcite with brown and white aragonite in one stone.
  • Onyx marble and Mexican onyx. Minerals.net defines onyx marble as banded travertine or tufa in the mineral form of aragonite or calcite. The label alone does not tell you which; the planned Mexico origins guide covers that material.
  • Satin spar. Usually fibrous gypsum, but Minerals.net notes the name occasionally describes fibrous calcite or aragonite.
  • "Aragonite" crystals that are calcite inside. Minerals.net notes some aragonite crystals sold to collectors are calcite pseudomorphs after aragonite: a six-sided outline with calcite's cleavage on a broken face.

How do prices compare?

Both are common and mostly inexpensive. Minerals.net says most calcite specimens are cheap except for those of exceptional size and crystal form or from classic occurrences, such as the very large, lustrous golden-brown crystals of the Elmwood Mine near Carthage, Tennessee. Aragonite's prices follow the same logic: sharp, intact twinned clusters from Morocco's Tazouta mine and classic Spanish trillings from Molina de Aragón command more than massive material.

For cut stones the picture reverses. The IGS notes that facetable aragonites are almost always very small, while calcite occurs in huge, transparent masses; the largest known cut aragonite, from Bilin in the Czech Republic, weighs 110 carats. Neither is hard enough for daily-wear jewelry.

In crystal traditions, calcite is linked with clarity and aragonite with grounding. Those associations come from custom, not mineralogy, and should not move the price.

Choosing aragonite or calcite specimens

Ask for a close photo of a broken edge or the base of the specimen. Flat, stepped rhomb faces mean calcite; curved, uneven breaks mean aragonite. For aragonite clusters, count intact points, because the prisms chip; for calcite, check edges and faces for cleavage cracks that could split later. If you buy a polished piece under either name, the label is the seller's opinion unless they can say how they know.

  • Star Aragonite (Morocco): small Moroccan twinned aragonite, the place to see the six-sided outline and multi-directional end grooves with a loupe.
  • Aragonite Cluster (MD): a medium Moroccan cluster, about 33 grams and 1 by 1.5 by 2 inches, listed as orthorhombic aragonite; useful for the copper coin test on its base.
  • Dogtooth Calcite Cluster with Mica: a 223 gram cluster of scalenohedral calcite, the classic calcite shape that aragonite never takes.
  • Optical Calcite: clear to honey calcite that splits into rhombs and doubles a printed line, a clean demonstration of cleavage and birefringence.
  • Caribbean Calcite Crystal Cut Corner Cube Polished: blue calcite with white and brown banding, the hybrid material often sold as blue aragonite.
  • White Aragonite: white aragonite for comparing luster and fracture against the calcite pieces above.

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Frequently asked questions

Is aragonite a type of calcite?

No, although they share a formula. Both are calcium carbonate, but calcite stacks its atoms in a trigonal structure and aragonite in an orthorhombic one, so they are separate mineral species called polymorphs. Aragonite is harder and denser, lacks calcite's three-way cleavage, and slowly converts to calcite over geological time.

Does vinegar tell aragonite from calcite?

No. Both fizz in dilute hydrochloric acid, cold or warm, and both react weakly with vinegar. Acid only confirms a carbonate. To separate the two, look at crystal shape and at how a piece breaks: calcite cleaves into rhombs, while aragonite fractures unevenly and grows as needles, sprays and six-sided twins.

Is blue aragonite really aragonite?

Sometimes. Natural blue aragonite exists, for example from Laurium in Greece. But much bright blue material sold under the name is Caribbean calcite, a mix of blue calcite with brown and white aragonite, and some cheap blue pieces are dyed. Ask the seller for the locality and how the identification was made.

Why do some aragonite crystals break like calcite?

They may be calcite pseudomorphs after aragonite. Aragonite is unstable at the surface and slowly converts to calcite, sometimes keeping its original six-sided outline. A crystal that looks like an aragonite twin outside but shows flat rhombohedral cleavage on a broken face has likely made that change.

Which is more fragile, aragonite or calcite?

Calcite is softer, at hardness 3 with three perfect cleavages, so it scratches and splits more easily. Aragonite is a little harder but often grows as thin needles and delicate sprays that snap at a touch. Handle both by the base, keep them dry and away from acids, and store them apart from harder stones.

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