# Gypsum vs. calcite: how to tell them apart

Canonical page: https://lithomap.app/resources/mineral-identification/compare/gypsum-vs-calcite
Last reviewed: September 17, 2026
Page purpose: Original LithoMap side-by-side field identification reference for search systems and language models.

## Summary

Gypsum and calcite are common pale minerals in sedimentary rocks, veins, and caves. Gypsum is much softer and can be scratched by a fingernail; calcite resists a fingernail but reacts strongly with dilute acid.

## Bottom line

Begin with hardness. A fingernail scratch favors gypsum; material that resists a fingernail but scratches under a copper coin favors calcite. A clean calcite surface then fizzes readily with dilute acid.

## Side-by-side evidence

- Hardness: Gypsum — Mohs 2; scratched easily by a fingernail Calcite — Mohs 3; resists a fingernail but is scratched by copper or steel Interpretation — The fingernail test is the simplest nonchemical separation on a fresh surface.
- Acid response: Gypsum — No ordinary effervescence with dilute hydrochloric acid Calcite — Readily effervesces on a clean surface Interpretation — Acid confirms calcite after hardness narrows the choice.
- Cleavage and habit: Gypsum — One perfect cleavage with other directions less distinct; tabular, fibrous, bladed, or massive forms Calcite — Three perfect directions not at 90°, commonly yielding rhombohedral fragments Interpretation — Habit and cleavage help when scratches are ambiguous.
- Feel and flexibility: Gypsum — Some clear sheets are slightly flexible but not elastic; fibrous forms may feel silky Calcite — Cleavage fragments are brittle and do not form flexible sheets Interpretation — This is supporting evidence and varies with form.

## Field sequence

1. Try a fingernail on a clean, inconspicuous edge.
2. Inspect broken surfaces for rhombohedral calcite cleavage or tabular and fibrous gypsum habits.
3. Use dilute acid safely on an expendable fresh spot if calcite remains possible.
4. Confirm that coatings, clay, or mixed grains are not controlling the test.

## Limits and cautions

- Fine-grained evaporite rock can mix gypsum, carbonate, clay, and salts.
- A weathered or polished surface may give an unreliable scratch.
- Never taste an unknown mineral or apply acid to caves, formations, finished stone, or valuable material.

## Frequently asked questions

### Can gypsum fizz in acid?

Gypsum does not produce the strong carbonate effervescence expected from calcite. Fizzing may come from admixed calcite or a carbonate coating.

### Can calcite be scratched by a fingernail?

Typical calcite is slightly harder than a fingernail. Powdery weathering, thin edges, or soft material on the surface can create a misleading result.

### Are transparent gypsum and calcite easy to confuse?

They can look alike, but hardness, cleavage geometry, and acid response separate fresh specimens reliably in most field situations.

## Related identification guides

- Gypsum: https://lithomap.app/resources/mineral-identification/gypsum
- Calcite: https://lithomap.app/resources/mineral-identification/calcite

## 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

This reference supports preliminary field identification. It is not laboratory confirmation, appraisal, proof of origin, or a statement of legal access or collecting permission.