Gypsum Identification Guide

What Gypsum Looks Like

Gypsum (CaSO₄·2H₂O) is a soft, common evaporite mineral that takes many forms. Selenite is clear, colorless, glassy crystals or plates; satin spar is fibrous with a silky, cat's-eye sheen; alabaster is fine-grained and massive; desert rose is a rosette of bladed crystals with sand. Colors are usually white or colorless but can be grey, yellow, pink, or brown from impurities.

• Color: colorless, white, grey, yellow, pink, brown
• Transparency: transparent (selenite) to translucent or opaque (alabaster)
• Luster: vitreous, pearly on cleavage, silky (satin spar)
• Habit: tabular/bladed crystals, fibrous veins, granular masses, rosettes

Step-by-Step Field-ID Checklist

1. Scratch with a fingernail. Gypsum (hardness 2) is easily scratched by a fingernail — the single most decisive field test.
2. Try to peel a flake. Selenite splits into thin, flexible (but not elastic) sheets along its perfect cleavage.
3. Check the sheen. Silky fibrous luster indicates satin spar.
4. Test acid. Gypsum does not fizz with dilute acid — separating it from soft carbonates.
5. Feel the weight. It is light, with a low specific gravity (~2.3).

Key Diagnostic Tests

• Mohs hardness: 2 — fingernail scratches it; it cannot scratch a coin.
• Streak: white.
• Cleavage: one perfect cleavage (plus two poorer), yielding thin flexible flakes.
• Fracture: splintery to conchoidal across cleavage.
• Specific gravity: ~2.3 — light.
• Acid: no effervescence (distinguishes from calcite).
• Water: slightly soluble; selenite can be slightly bent but is not elastic like mica.

Common Look-Alikes and How to Tell Them Apart

• Calcite: scratches more (hardness 3), fizzes vigorously in acid, and has rhombohedral cleavage; gypsum is softer and acid-inert.
• Halite (rock salt): tastes salty, has cubic cleavage, and is harder (2.5); gypsum has no taste and one main cleavage.
• Mica (muscovite): flakes are thin but elastic (snap back); gypsum flakes are flexible but stay bent.
• Anhydrite (CaSO₄): harder (3–3.5), denser, three cleavages at right angles; gypsum is softer and hydrated.
• Quartz (in clear crystals): far harder (7) and scratches glass; gypsum does not.

The make-or-break test is the fingernail scratch (H=2) combined with no acid reaction.

Where Gypsum Is Found

Gypsum forms by evaporation of seawater and lake brines, so it occurs in thick evaporite beds worldwide — famously the White Sands gypsum dunes (New Mexico), the Cave of the Crystals (Naica, Mexico) with giant selenite, plus extensive deposits in Spain, Italy, and across the U.S. Midwest. It also grows in clays as desert roses.

Quick Field Summary

A light, soft mineral you can scratch with a fingernail, that splits into flexible non-elastic flakes and does not fizz in acid, is gypsum — not calcite (fizzes), halite (salty), or mica (elastic).