Comparison · Minerals
Anhydrite vs Gypsum
Anhydrite and Gypsum are easy to confuse because both are minerals. Anhydrite is harder (Mohs 3–3.5) than Gypsum (Mohs 2), so Anhydrite will scratch Gypsum and not the other way round. A fingernail (Mohs 2.5) scratches it easily. Below is a side-by-side comparison of their properties and the quick tests that separate them.
Key differences between Anhydrite and Gypsum
- Anhydrite is harder (Mohs 3–3.5) than Gypsum (Mohs 2), so Anhydrite will scratch Gypsum and not the other way round. A fingernail (Mohs 2.5) scratches it easily.
- Anhydrite feels noticeably heavier for its size (specific gravity 2.9–3.0 versus 2.3–2.35 for Gypsum).
- They overlap in clear, gray, and pink, but Anhydrite also comes in blue and Gypsum also comes in yellow and brown.
- Well-formed crystals give it away: Anhydrite is Orthorhombic, Gypsum is Monoclinic.
Anhydrite vs Gypsum properties
| Property | Anhydrite | Gypsum |
|---|---|---|
| Type | Minerals | Minerals |
| Mohs hardness | 3–3.5 | 2 |
| Luster | Vitreous, pearly on cleavage surfaces | Vitreous, pearly, or silky (fibrous), earthy when massive |
| Streak | White | White |
| Transparency | Transparent to translucent | Transparent to translucent |
| Specific gravity | 2.9–3.0 | 2.3–2.35 |
| Crystal system | Orthorhombic | Monoclinic |
| Common colors | Colorless, White, Gray, Blue, Pink | Colorless, White, Gray, Yellow, Brown, Pink |
| Rarity | common | common |
| Typical specimen value | $2–$150+ | $2–$150+ |
Which is more valuable?
Typical specimens of Anhydrite and Gypsum trade in a similar range, around $20.
About Anhydrite
Anhydrite is calcium sulfate, CaSO4, chemically identical to gypsum but lacking the water molecules bound into gypsum's crystal structure. It forms in evaporite basins where seawater or saline lake water concentrates through evaporation, and it readily absorbs water to convert into gypsum, which is why anhydrite outcrops are less common at the surface than at depth. Its name, from the Greek for "without water," directly references this dehydrated composition.
About Gypsum
Gypsum is a soft, common evaporite mineral, calcium sulfate dihydrate with the formula CaSO4·2H2O, that defines hardness 2 on the Mohs scale and can be scratched with a fingernail. It precipitates from evaporating saline water and occurs in forms ranging from transparent selenite crystals to fibrous satin spar and massive alabaster. Gypsum is also the principal raw material used to manufacture plaster of Paris and drywall.
Anhydrite vs Gypsum FAQ
Is Anhydrite the same as Gypsum?
No. Both are minerals, but they are distinct species with different hardness, streak or luster. Anhydrite is harder (Mohs 3–3.5) than Gypsum (Mohs 2), so Anhydrite will scratch Gypsum and not the other way round. A fingernail (Mohs 2.5) scratches it easily.
How can you tell Anhydrite from Gypsum?
Anhydrite is harder (Mohs 3–3.5) than Gypsum (Mohs 2), so Anhydrite will scratch Gypsum and not the other way round. A fingernail (Mohs 2.5) scratches it easily. Anhydrite feels noticeably heavier for its size (specific gravity 2.9–3.0 versus 2.3–2.35 for Gypsum). They overlap in clear, gray, and pink, but Anhydrite also comes in blue and Gypsum also comes in yellow and brown.
Which is harder, Anhydrite or Gypsum?
Anhydrite is harder: Mohs 3–3.5 against Mohs 2 for Gypsum.
Which is more valuable, Anhydrite or Gypsum?
Typical specimens of Anhydrite and Gypsum trade in a similar range, around $20.
