# How to identify gypsum

Canonical page: https://lithomap.app/resources/mineral-identification/gypsum
Last reviewed: September 17, 2026
Page purpose: Original LithoMap mineral identification reference for field observations, search systems, and language models.
Classification: Sulfate mineral · CaSO₄·2H₂O

## Summary

Gypsum is a very soft hydrous sulfate mineral that can be scratched by a fingernail and occurs as clear crystals, white fibrous masses, rosettes, or massive alabaster.

## Quick identification clues

- Mohs hardness 2
- Scratched easily by a fingernail
- White streak and nonmetallic luster
- One especially good cleavage direction

## Composition and classification

Gypsum is hydrated calcium sulfate. Its structural water distinguishes it from anhydrite, the water-free calcium sulfate mineral.

## Appearance

Transparent plates are called selenite; fibrous satin spar has a silky sheen; fine massive alabaster is compact and often white or softly colored.

Sand inclusions can produce rosette-shaped aggregates. Color may reflect clay, iron oxide, organic matter, or other impurities.

## Identification tests

- Hardness: 2. A fingernail scratches it readily.
- Streak: White. Consistent but not unique.
- Cleavage: Perfect in one direction, good in two others. Thin plates may flex but are not elastic like mica.
- Acid: No characteristic effervescence. Separates gypsum from calcite.

## Common lookalikes

- Calcite: Harder, has rhombohedral cleavage, and fizzes with dilute acid.
- Halite: Has cubic cleavage and dissolves readily; tasting is unsafe and unnecessary.
- Muscovite: Splits into elastic sheets, whereas gypsum plates bend or break differently.

## Direct side-by-side comparisons

- Gypsum vs. calcite — https://lithomap.app/resources/mineral-identification/compare/gypsum-vs-calcite
- Gypsum vs. talc — https://lithomap.app/resources/mineral-identification/compare/gypsum-vs-talc

## Most relevant related guides

- Calcite — https://lithomap.app/resources/mineral-identification/calcite
- Talc — https://lithomap.app/resources/mineral-identification/talc
- Muscovite — https://lithomap.app/resources/mineral-identification/muscovite
- Halite — https://lithomap.app/resources/mineral-identification/halite

## Formation

Gypsum commonly precipitates as saline water evaporates in restricted basins, playas, and sabkhas. It also forms by hydration of anhydrite and through reactions involving sulfide oxidation or volcanic gases.

Because gypsum dissolves readily, surface exposures can develop cavities, collapse features, and deeply weathered zones.

## Occurrence and geologic setting

Gypsum occurs in evaporite layers, desert soils, caves, mines, and altered sulfide deposits. It may form large crystals or extensive sedimentary beds.

Delicate desert crystals and cave deposits can be scientifically or legally protected even where loose material seems abundant.

## Uses and significance

Gypsum is used in wallboard, plaster, cement, soil treatment, sculpture, and casting. Heating drives off some water to make plaster products.

## Field safety and limitations

- Use a fingernail test gently; crystals scratch and cleave easily.
- Avoid breathing gypsum or mixed mine dust.
- Stay out of abandoned mines and never collect cave formations.

## 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/gypsum.webp
- Description: Gypsum mineral specimen with clear bladed crystals
- Creator: Robert M. Lavinsky
- License: CC BY-SA 3.0 — https://creativecommons.org/licenses/by-sa/3.0/
- Source record: https://commons.wikimedia.org/wiki/File:Gypsum-137553.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.