# How to identify calcite

Canonical page: https://lithomap.app/resources/mineral-identification/calcite
Last reviewed: September 17, 2026
Page purpose: Original LithoMap mineral identification reference for field observations, search systems, and language models.
Classification: Carbonate mineral · CaCO₃

## Summary

Calcite is a relatively soft carbonate mineral recognized by Mohs hardness 3, three-direction rhombohedral cleavage, and vigorous reaction with dilute acid on a fresh surface.

## Quick identification clues

- Mohs hardness 3
- Three perfect cleavage directions forming rhombs
- Strong effervescence with dilute acid
- Commonly white, clear, gray, cream, or stained

## Composition and classification

Calcite is calcium carbonate. Magnesium, iron, manganese, and other elements may substitute in small amounts, and inclusions can change color or transparency.

## Appearance

Calcite has many crystal habits, including rhombohedra and scalenohedra, and also occurs massive, granular, fibrous, or earthy.

Clear cleavage fragments may show double refraction, making a line viewed through the crystal appear doubled. Weathered calcite can be dull or chalky.

## Identification tests

- Hardness: 3. Scratched by a copper coin or steel point; harder than a fingernail.
- Streak: White. Helpful for colored samples but not unique.
- Cleavage: Perfect in three directions, not at 90°. Produces repeated rhombohedral fragments.
- Acid: Strong fizz on a fresh surface. A defining carbonate test when performed safely.

## Common lookalikes

- Dolomite: Usually reacts weakly on an unpowdered surface and is slightly harder.
- Quartz: Hardness 7, no cleavage, and no acid reaction.
- Gypsum: Softer than a fingernail and does not effervesce with acid.

## Direct side-by-side comparisons

- Quartz vs. calcite — https://lithomap.app/resources/mineral-identification/compare/quartz-vs-calcite
- Gypsum vs. calcite — https://lithomap.app/resources/mineral-identification/compare/gypsum-vs-calcite
- Calcite vs. dolomite — https://lithomap.app/resources/mineral-identification/compare/calcite-vs-dolomite
- Barite vs. calcite — https://lithomap.app/resources/mineral-identification/compare/barite-vs-calcite

## Most relevant related guides

- Dolomite — https://lithomap.app/resources/mineral-identification/dolomite
- Gypsum — https://lithomap.app/resources/mineral-identification/gypsum
- Quartz — https://lithomap.app/resources/mineral-identification/quartz
- Barite — https://lithomap.app/resources/mineral-identification/barite

## Formation

Calcite forms in marine and freshwater sediments, shells and skeletons, caves, hydrothermal veins, altered rocks, and metamorphic marble.

It precipitates from water and also recrystallizes during burial or metamorphism, so the same mineral can record very different environments.

## Occurrence and geologic setting

Calcite is the principal mineral in most limestone and marble and is common in veins, geodes, travertine, cave deposits, and fossil material.

Optical clarity, color, and crystal shape vary. A hand-sample identification does not determine whether a specimen is natural, treated, or commercially valuable.

## Uses and significance

Calcite-bearing rock supplies cement, lime, aggregate, agricultural lime, fillers, and dimension stone. Crystalline calcite also has optical and collecting uses.

## Field safety and limitations

- Dilute acid can damage specimens and structures; use only where permitted with eye protection.
- Never remove cave formations.
- Do not lick or taste an unknown specimen.

## Authoritative sources

- Minerals and their physical properties — U.S. National Park Service: https://www.nps.gov/subjects/geology/minerals.htm
- Mohs Hardness Scale — U.S. National Park Service: https://www.nps.gov/articles/mohs-hardness-scale.htm
- Natural Gemstones: Mineral Gemstones — U.S. Geological Survey: https://pubs.usgs.gov/gip/gemstones/mineral.html

## Specimen photograph

- Image: https://lithomap.app/_delivery/identification/minerals/calcite.webp
- Description: Calcite mineral specimen with pale translucent crystals
- Creator: Raimond Spekking
- License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/
- Source record: https://commons.wikimedia.org/wiki/File:Calcite,_Mexico-8827.jpg

This reference supports preliminary field identification. It is not a laboratory determination, appraisal, hazard assessment, proof of origin, or statement of land ownership, legal access, or collecting permission.