Rock Gypsum Identification Guide

What Rock Gypsum Looks Like

Rock gypsum is a sedimentary evaporite rock composed mainly of the mineral gypsum (hydrated calcium sulfate, CaSO₄·2H₂O). It is usually white, gray, pink, or tan, with a dull, earthy to slightly pearly or satiny appearance. Texture ranges from massive and sugary to fibrous (satin spar) or the granular "alabaster" form. It forms as thick beds where seawater or lake water evaporated, and it is famously so soft you can scratch it with a fingernail.

Step-by-Step Field ID Checklist

1. Try the fingernail test. A fingernail scratches it — this alone rules out nearly all common rocks.
2. Note the color and luster. White-to-pale, dull to satiny; fibrous "satin spar" has a silky sheen.
3. Feel the weight. Light to moderate; not dense.
4. Check the acid response. It does not fizz in dilute HCl (it is a sulfate, not a carbonate).
5. Look at the setting. Bedded with other evaporites (salt, anhydrite) in basin sequences.

Key Diagnostic Tests

• Mohs hardness: 2 — scratched by a fingernail; this is the single most diagnostic test.
• Streak: White.
• Cleavage: Single mineral gypsum has one perfect cleavage; the rock often breaks earthy/granular.
• Acid: No effervescence in dilute HCl (separates it from limestone/dolomite).
• Density: ~2.3 g/cm³, low.
• Solubility: Slightly soluble in water; not salty to taste (unlike halite).

Common Look-Alikes and How to Tell Them Apart

• Anhydrite (CaSO₄): The dehydrated form — harder (3–3.5), so a fingernail will not scratch it; often denser and more crystalline. Hardness is the separator.
• Rock salt (halite): Tastes salty and has cubic cleavage; gypsum is tasteless. (Taste-test a tiny clean fragment cautiously.)
• Limestone/chalk: Fizzes in acid, and limestone is harder (3) than gypsum; gypsum does not react and is softer.
• Marble: Hard (3, but crystalline), fizzes in acid; gypsum is soft and acid-inert.
• Talc/soapstone: Also fingernail-soft but greasy/soapy feel and green-gray; gypsum is not soapy.

Where Rock Gypsum Is Found

Rock gypsum forms in evaporite basins — shallow seas, sabkhas, and saline lakes that dried up. Major deposits occur in the US (Texas, Oklahoma, New Mexico, Kansas, Michigan), Mexico, Spain, and across many sedimentary basins worldwide, often interbedded with shale, anhydrite, and halite. Look for thick pale beds in arid-region sedimentary sequences and quarry walls.

Formation and Collecting Notes

Rock gypsum precipitates in evaporite basins — restricted seas, coastal sabkhas, and saline lakes — where evaporation concentrates dissolved calcium sulfate until it crystallizes, often in a predictable sequence with carbonate below and halite above. Buried gypsum can dehydrate to anhydrite at depth and rehydrate back to gypsum near the surface, so the two minerals are commonly interbedded.

The fingernail-hardness test is the workhorse here: almost no other common bedded rock is that soft, so a white-to-pinkish bed you can scratch with a nail, that does not fizz in acid and is not salty, is very likely rock gypsum. Recognize its varieties in the field — satin spar (silky fibrous veins), alabaster (fine massive carving stone), and selenite (clear cleavable crystals). Because gypsum is slightly water-soluble, outcrops often show solution pits, smooth dissolved surfaces, and sinkholes. Handle alabaster gently when carving and keep specimens dry, since prolonged moisture can dull and degrade soft gypsum surfaces.