Crystal Harvest Field Guide

Calcite: The Complete Guide

The mineral that builds limestone, marble, caves, and seashells. It revealed double refraction to science and may have guided Viking ships. It forms more crystal shapes than almost any other mineral.

Quick facts

CaCO3Calcium carbonate. Trigonal crystal system.
3 on MohsCalcite is the reference mineral for 3. A steel knife scratches it easily.
Perfect cleavageSplits cleanly in 3 directions into rhombs, with faces meeting at about 75°.
SG 2.71Refractive indices about 1.49 and 1.66, a very large difference.
Double refractionClear calcite splits light into two rays, so anything viewed through it appears doubled.
Fizzes in acidReacts with dilute acid, even vinegar, releasing carbon dioxide bubbles.
1,000+ formsOver 1,000 crystallographic forms and roughly 800 described habits.
Often fluorescentCan glow red, orange, blue, yellow, and more under UV, often thanks to manganese.

Calcite in history and science

Calcite has shaped physics, navigation, and warfare as well as landscapes.

  1. 79 CEThe name traces to the Latin calx, “lime.” Pliny the Elder wrote about it.
  2. 1592An English warship sinks off Alderney in the Channel Islands. Divers later find a calcite crystal less than a meter from the ship’s navigation dividers. Researchers proposed in 2013 that it served as an optical compass.
  3. 1600s–1900sHelgustaðir, near Eskifjörður in Iceland, produces the world’s largest and clearest optical calcite, which gave “Iceland spar” its name.
  4. 1669Rasmus Bartholin describes double refraction in Iceland spar, the first scientific account of the effect.
  5. 1690s–1820sChristiaan Huygens uses Iceland spar to support his wave theory of light, and Augustin Fresnel fully explains double refraction in the 1820s.
  6. 1828William Nicol invents the Nicol prism from split and re-cemented Iceland spar, producing polarized light for microscopes and polarimeters.
  7. 1942–1943The US declares optical calcite a strategic material for gunsights, including the Polaroid ring sight. Demand peaks around 45,000 lb a year, supplied from California, Montana, and Mexico.
  8. 1967Thorkild Ramskou suggests the Viking “sunstone” of the sagas was Iceland spar.
  9. 1975Helgustaðir becomes a nature reserve, and removing crystals is forbidden.
  10. 2011Researchers at the University of Rennes show that Iceland spar can locate the sun within a few degrees, even at twilight.

How calcite forms

Calcite is one of the most common minerals at Earth’s surface. It forms from living things, from cave water, from hot springs, and from hydrothermal fluids.

  1. Life builds it. Oysters, brachiopods, sea urchins, and even trilobite eyes are made of calcite. Chalk is made of microscopic calcite plates from plankton, and the Cretaceous Period is named after chalk (Latin creta).
  2. Rocks pile up. Shells and lime mud harden into limestone, which heat and pressure can turn into marble.
  3. Caves dissolve. Rainwater carrying carbon dioxide forms weak carbonic acid that dissolves limestone and hollows out caves.
  4. Caves decorate. Inside the cave, dripping water releases CO2 and deposits calcite as stalactites, stalagmites, flowstone, and draperies. See how caves are made.
  5. Springs deposit travertine. Calcium-rich spring water, especially hot springs, builds travertine terraces like Pamukkale and Mammoth Hot Springs. Rome’s Colosseum is built of it.
  6. Crystals grow in pockets. Hydrothermal fluids grow sharp crystals in cavities, as in the lead-zinc mines of the Tri-State district.

Calcite has a close relative: aragonite has the same chemistry, CaCO3, with a different structure. Aragonite is slightly less stable and slowly turns into calcite over tens of millions of years. Read more in aragonite vs. calcite.

Double vision: calcite and light

Put a clear calcite rhomb on a printed word and you see it twice. Calcite’s two refractive indices are so different that light entering the crystal splits into two rays that travel at different speeds and bend by different amounts.

Turn the crystal and one image stays still while the other circles around it. The two rays are also polarized at right angles to each other. That property made calcite the heart of early polarizing microscopes, and possibly of Viking navigation: a sunstone could reveal the sun’s position through cloud by comparing the two images.

Read more about the physics in birefringence.

Diagram of a calcite rhomb splitting one light ray into two rays that exit parallel but offset, producing a double image1 ray inordinary rayextraordinaryray2 rays out
One ray in, two rays out: the double image.

Famous calcite localities

Calcite occurs almost everywhere, but these places are famous for their specimens.

Helgustaðir, Iceland

Eskifjörður

The original Iceland spar mine and source of most classic museum optical calcite. Now a protected reserve.

Elmwood Mine, Tennessee

Carthage, Smith County

Golden-brown dogtooth crystals, sometimes over a foot long, with purple fluorite, sphalerite, and barite.

Tri-State district

Missouri, Kansas, Oklahoma

Lead-zinc country from the 1850s to 1970. Joplin’s Crystal Cave, found in 1893, was lined with calcite crystals up to 22 inches long.

Franklin, New Jersey

Sussex County

Calcite glowing orange-red under shortwave UV beside green willemite. See Franklin and Sterling Hill.

Rossie, New York

St. Lawrence County

Classic pink to purple calcite crystals.

Chihuahua, Mexico

Northern Mexico

A major modern source of optical Iceland spar.

Ohio

Northwest quarries

Golden “honey” calcite in Devonian carbonate rock. See Ohio honey calcite.

Kolwezi, DR Congo

Katanga

Neon-pink cobaltoan calcite, a classic of the 1980s–90s.

Record breakers

  • Largest calcite crystal: an Iceland spar crystal from Iceland reported at about 7 × 7 × 2 m and roughly 250 tonnes.
  • Joplin’s Crystal Cave: calcite crystals up to 22 inches lined the walls. The cave was sealed around 1932 and has since flooded.

Varieties and habits

Iceland spar

Water-clear rhombs showing strong double refraction.

Dogtooth

Sharp, pointed scalenohedral crystals.

Nailhead

Crystals capped with flat rhombohedral tops, like nail heads.

Honey / golden

Yellow to amber, usually colored by iron.

Cobaltoan

Hot pink to purple from trace cobalt.

Manganoan (“mangano”)

Pink from manganese, often glowing under UV. Part of a series toward rhodochrosite, not a separate species.

Travertine and onyx marble

Banded spring and cave calcite, sold as “Mexican onyx.” See Ilion Gorge travertine.

Cave pearls

Smooth round calcite balls built in layers in cave pools.

“Onyx,” dyed calcite, and trade names

Calcite is soft and cheap to dye, so it turns up under many borrowed names.

Sold asUsually isQuick test
“Mexican onyx,” “onyx marble,” “limestone onyx”Banded calcite or travertine. True onyx is chalcedony (quartz).A drop of vinegar fizzes on calcite, not onyx. A knife scratches calcite (3), not onyx (7).
“Blue onyx,” “green onyx,” “Caribbean calcite”Often dyed banded calciteColor pooled in cracks and bands, plus the vinegar test
Bright “orange,” “blue,” or “green calcite”Often massive, uncrystallized calcite, natural or dyedAsk about locality and treatment

Only acid-test an inconspicuous spot, and rinse right away.

Myths vs. facts

Myth: Vikings definitely navigated with calcite.

Fact: It’s a plausible, experimentally tested idea, but no calcite crystal has been found at a Viking site. The only shipwreck find is from 1592.

Myth: “Mexican onyx” is onyx.

Fact: It is banded calcite, hardness 3. True onyx is quartz, hardness 7.

Myth: The giant crystals of Naica are calcite.

Fact: They are gypsum (selenite), up to about 11 m long.

Myth: Blackboard chalk is calcite.

Fact: Sidewalk and many blackboard chalks are mostly gypsum. Natural chalk rock is calcite.

Myth: Iceland spar only comes from Iceland.

Fact: The name comes from Iceland, but optical calcite has also been mined in Mexico, New Mexico, Montana, and California.

Myth: Calcite and aragonite are the same mineral.

Fact: Same chemistry, different structure. They are polymorphs.

Myth: Calcite makes a good everyday ring stone.

Fact: It is too soft, cleaves easily, and is sensitive to acid.

Care and display

No acids

Vinegar, lemon, and many household cleaners etch calcite. Use plain water only.

Gentle handling

At hardness 3 with perfect cleavage, calcite scratches and splits easily. Avoid ultrasonic cleaners and knocks.

Heat and solvents

Keep away from heat and solvents, which can crack or dull it.

Light

Some colored calcite can fade in strong sunlight. Display fluorescent pieces with a UV lamp, not the sun.

Calcite in the shop

Smithsonite with Calcite — Laurion, Greece
Smithsonite with Calcite — Laurion, Greece$45.10
Calcite Crystal with Dark Blade Inclusions — New York
Calcite Crystal with Dark Blade Inclusions — New York$34.00
Blue aragonite specimen from the Rio Tinto Mine, Spain on a white background beside a caliper for scale
Blue Aragonite — Rio Tinto Mine, Spain$31.16
Hand holding an orange botryoidal calcite from New York, rounded bubble-like tan crust over grey rock
Orange Botryoidal Calcite — New York$11.48
Hand holding a New Jersey specimen of honey calcite and quartz crystals with dark metallic sulfides
Calcite & Quartz with Sulfides — New Jersey$29.52
Hand holding an elongated New Jersey specimen with a pale grey-green calcite crust over stilbite
Calcite on Stilbite — New Jersey$36.08
Blue aragonite from the Rio Tinto Mine, Spain, pale turquoise crust on dark rock, beside a caliper
Blue Aragonite — Rio Tinto Mine, Spain$42.64
Golden calcite crystal point from Ohio, honey coloured and translucent, shown beside a caliper
Golden Calcite Crystal Point — Natural Ohio Specimen$22.96
Three golden Ohio calcite pieces from Weston, Wood County arranged on white beside a caliper
Ohio Calcite Crystal Lot — 3 Pieces from Weston, Wood County$36.08
Quartz and calcite mineral specimen from Mexico shown on a white background beside a caliper
Quartz and Calcite Mineral Specimen – Mexico$26.24
See all calcite

Frequently asked questions

How do I tell calcite from quartz?

Calcite is hardness 3, so a knife scratches it, while quartz (7) scratches glass. Calcite also fizzes in acid and breaks into rhombs.

Why do I see double through clear calcite?

Its strong birefringence splits light into two rays that exit offset from each other, doubling the image.

Does calcite glow under UV light?

Often, yes. The color depends on locality and impurities. Franklin, NJ calcite glows orange-red under shortwave UV.

How do I clean calcite?

Plain water and a soft brush only. No vinegar, acids, ultrasonic cleaners, solvents, or heat.

Is my “blue onyx” real onyx?

Probably not. It is likely dyed or banded calcite. A tiny drop of vinegar on a hidden spot will fizz if it is calcite.

Can I collect Iceland spar at Helgustaðir?

No. The old mine is a nature reserve and removing crystals is forbidden.

What is calcite crystal good for?

Calcite is one of the most useful minerals in industry: it is the main ingredient of limestone and marble, the raw material for cement and lime, a soil conditioner in agriculture, a filler in paper and paint, and the active ingredient in calcium carbonate antacids. Its strong double refraction made clear Iceland spar essential for early optical instruments. For collectors, it forms more crystal shapes than almost any other mineral.

Is calcite expensive?

Usually not. Calcite is extremely common, so most pieces are inexpensive. Fine specimens, such as large clear crystals, rare colors (pink cobaltoan or manganoan calcite), or classic localities, can be valuable, and optical-grade Iceland spar has always commanded a premium.

What are the different types of calcite?

Common forms and varieties include Iceland spar (clear, optical-grade), dogtooth (pointed scalenohedral crystals), nailhead (flat-topped crystals), rhombohedral cleavage pieces, honey and golden calcite, cobaltoan and manganoan calcite (pink), optical, blue, green, and orange massive calcite, and banded forms like travertine and cave onyx.

Where is calcite found?

Almost everywhere. It makes up limestone and marble, lines caves as stalactites and stalagmites, fills veins and cavities in many rock types, and builds shells and coral. Classic specimen localities include Ohio, Missouri, Tennessee, and New York in the U.S., plus Mexico, England, China, and Iceland. See our calcite specimens.

How is calcite formed?

Calcite (calcium carbonate, CaCO3) crystallizes from water that carries dissolved calcium and carbonate, either when the water loses carbon dioxide, warms, or evaporates, or when living things like clams and corals build shells from it. It also forms from hot hydrothermal fluids in mineral veins.

What color is calcite?

Pure calcite is colorless or white. Small amounts of other elements or inclusions color it: iron gives yellow to brown and orange, manganese gives pink, cobalt gives vivid pink to magenta, and inclusions of other minerals give green, blue, or gray.

Keep reading

Sources and further reading

The Viking sunstone remains a hypothesis. This page presents it that way.

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