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Crystal Structure
Monoclinic CaSO₄·2H₂O.
External databases provide CIF (Crystallographic Information File) downloads + interactive 3D viewers. AMCSD: American Mineralogist Crystal Structure Database (free, RRUFF-hosted). COD: open community-curated database.
Elemental Composition (by mass)
| Element | Mass % | Visual |
|---|---|---|
| O Oxygen | 46.32% | |
| Ca Calcium | 29.01% | |
| S Sulfur | 23.21% | |
| H Hydrogen | 1.46% |
Computed from simplified end-member formula. Solid-solution series, water content, and trace substitutions cause real-world variation.
IMA Abbreviation (Whitney-Evans 2010)
Gp
→ Gypsum
Hydrated Ca sulfate
Standard symbol from American Mineralogist (Whitney & Evans, 2010). Used in thin-section labeling, phase diagrams, and IMA-style species records.
⏳ Long-term Aging & Care Timeline
surface cloudingyears
Trigger: humidity cycles
Intervention: Selenite surface may pale slightly with humidity cycling. Generally stable in moderate storage.
Pronunciation
/ˈdʒɪpsəm/
↔ JIP-sum
Latin/Greek gypsos
⚠ Safety & Handling
💧water-sensitivelow
Slowly dissolves in water.
Handling: Avoid wet cleaning. Dust with soft brush only.
Information provided in good faith. Consult local hazmat regulations for transport and disposal. Severely hazardous specimens may require special storage cabinets.
UV Fluorescence
SW (254 nm)
—
none
LW (365 nm)
Yellow
weak
Generally non-fluorescent; some U-bearing varieties weak.
SW = shortwave (germicidal lamp). LW = longwave (blacklight). Response varies with locality, trace impurities, and treatment.
Pseudomorph Relationships
Replaced by — this mineral commonly becomes:
Replaces — this mineral is often a pseudomorph after:
← Anhydrite hydration
Anhydrite (CaSO₄) hydrates to gypsum (CaSO₄·2H₂O) with ~63% volume increase — important in salt cap rock.
Salt dome cap rocks.
A pseudomorph (Greek "false form") is a mineral with the external shape of another species — the chemistry has changed but the crystal habit is inherited. › Full catalogue
Tenacity
Behavior:
flexible (sheets)
Under stress:
Folds without springing back
Selenite sheets bend; satin spar fibers part.
Luster
vitreous→pearly
Pearly to satiny on cleavage; silky for satin spar variety.
Notable localities (coordinates)
- Naica — Chihuahua, Mexico27.8500, -105.4900 › OpenStreetMap
All localities and full GeoJSON available at /wp-json/mmb/v1/localities-geo
Diaphaneity (Transparency)
transparent-to-translucent
Selenite is highly transparent; satin spar fibrous translucent.
Diagnostic Field Tests
Fingernail test→ Fingernail scratches it (Mohs 2)
Very soft; cleaves into thin sheets (selenite).
Water solubility→ Slightly soluble
Avoid water cleaning — minor solubility damages crystal faces.
⚠ Use dilute HCl (~10%) only on inconspicuous spots; rinse promptly. Smell-tests should be brief and ventilated. Taste-test ONLY halite/sylvite — never lead, arsenic, or sulfur minerals.
Specific Gravity
2.30–2.37
g/cm³
very light
Selenite, alabaster, satin spar varieties.
For comparison: water = 1.00, glass ≈ 2.5, quartz = 2.65, corundum ≈ 4.00, galena ≈ 7.50, gold ≈ 19.3.
Geological Setting
Environment:
sedimentary
Host rock:
evaporite, sedimentary geode
Companions:
Marine evaporite; Naica selenite from hydrothermal recrystallization.
Twinning Laws
Swallowtail twincontact
Two crystals joined at ~105°, forming a deeply notched "V". Common in selenite.
Fishtail twincontact
Similar to swallowtail but with sharper, narrower V — common at Naica.
Care notesDry brush. Soft (Mohs 2) — easily scratched. Never water-clean selenite. Full cleaning guide →
Formation eraEvaporite; Naica gypsum > 500ka.
Cleavage & Fracture
Cleavage:
perfect 1 perfect {010} + 2 distinct
Fracture:
splintery / fibrous
Perfect sheet cleavage; selenite variety.
Market availability: Common
Widely available in most dealer stocks. Specimens span all price tiers.
Collector tier: Solid Display
Reliable mid-tier display species. Easy to find in well-formed examples; broad locality diversity.
Mohs 2
Vickers (~) 75 HV
Knoop (~) 85 HK
Nickel–Strunz
7.CD.40Dana
29.06.03.01Element composition by mass
Formula: CaSO4 . 2H2O · molar mass: 154.15 g/mol
| O | 51.89% | |
| Ca | 26% | |
| S | 20.8% | |
| H | 1.31% |
Computed from atomic weights (IUPAC 2021). Site-occupancy groups (Fe,Mn) split equally.
Mohs Hardness
2
Gypsum sits at 2 on the Mohs scale — soft enough to be scratched by a fingernail.
Colors:
Streak
White
White
Crystal system
Monoclinic
Monoclinic
Type localityMontmartre, Paris, France (classical Plaster of Paris)
✓ I have this★ WishlistView my collection →
Discovery Known since antiquity
SulfatesSulfates – Hydrous
TL;DR · 1 min read
Gypsum is the Mohs-2 reference mineral and one of the most varied collector species. Its varieties span from the world's largest crystals (Naica selenite, 11+ meters) through the silky satin spar of stalactite caverns to the desert-rose rosettes of arid sand floors.
Text size:AAA
Gypsum is the Mohs-2 reference mineral and one of the most varied collector species. Its varieties span from the world's largest crystals (Naica selenite, 11+ meters) through the silky satin spar of stalactite caverns to the desert-rose rosettes of arid sand floors. Gypsum defines the second step of the Mohs hardness scale and dissolves slowly in water, producing some of the most fragile yet visually arresting display specimens.
Notable Varieties #
- Selenite (transparent crystals; the Naica giants)
- Satin spar (silky fibrous; cave deposits)
- Alabaster (massive granular; carving stone)
- Desert rose (rosette intergrowths in sand)
- Swallowtail twins (V-shaped contact twins)
The Chinese Angle #
Chinese gypsum is industrially significant (Hunan, Hubei, Inner Mongolia) but specimen-grade material reaches the international market sporadically. Xinjiang produces some satin spar and selenite blades; Hunan stalactite caves yield occasional botryoidal satin spar. Most collector gypsum still flows from Mexico, Pakistan, Morocco, and the United States.
Test at home — what scratches what
Will scratch your specimen:
💅 Fingernail (Mohs 2.5) · 🪙 Copper coin (US penny) (Mohs 3.5) · 🔪 Steel pocket knife (soft steel) (Mohs 5) · 🥃 Glass plate (Mohs 5.5) · 🔧 Steel file / hardened steel (Mohs 6.5) · ⚙ Sharp steel needle / quartz scratch (Mohs 7)
💅 Fingernail (Mohs 2.5) · 🪙 Copper coin (US penny) (Mohs 3.5) · 🔪 Steel pocket knife (soft steel) (Mohs 5) · 🥃 Glass plate (Mohs 5.5) · 🔧 Steel file / hardened steel (Mohs 6.5) · ⚙ Sharp steel needle / quartz scratch (Mohs 7)
Your specimen will scratch:
👆 Talc dust (Mohs 1)
👆 Talc dust (Mohs 1)
Always test on an inconspicuous edge first. Save the test for unimportant specimens — better to use a streak plate or knowledge of locality + paragenesis.
Cite this entry
APA
MyMineralBox Editorial Team. (2026). Gypsum. My Mineral Box. Retrieved May 23, 2026, from https://mymineralbox.com/mineral-encyclopedia/minerals/gypsum/
MLA
MyMineralBox Editorial Team. "Gypsum." My Mineral Box, 2026, https://mymineralbox.com/mineral-encyclopedia/minerals/gypsum/. Accessed May 23, 2026.
Chicago
MyMineralBox Editorial Team. "Gypsum." My Mineral Box. Last modified May 4, 2026. https://mymineralbox.com/mineral-encyclopedia/minerals/gypsum/.
BibTeX
@misc{mmb_gypsum,
author = {{MyMineralBox Editorial Team}},
title = {{Gypsum}},
year = {2026},
publisher = {My Mineral Box},
url = {https://mymineralbox.com/mineral-encyclopedia/minerals/gypsum/},
urldate = {2026-05-23}
}
