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Gypsum vs Calcite: Fingernail, Cleavage and Acid Tests

Gypsum and calcite commonly appear as white, colourless, translucent, or massive material. Gypsum is softer and normally scratches with a fingernail, while calcite usually does not. Calcite also effervesces readily in cool dilute acid; gypsum does not produce bubbles. Try a fingernail on an inconspicuous edge. Gypsum, Mohs 2, scratches; calcite, Mohs 3, normally resists. Calcite has three rhombohedral cleavage directions and readily fizzes in dilute acid, while gypsum has one especially good cleavage and no effervescence. Try a fingernail on a small inconspicuous edge. A definite groove strongly supports gypsum. Rotate the specimen under light. One dominant sheet-like cleavage supports gypsum; repeated rhombohedral planes support calcite. If trained and equipped, use one drop of cool dilute acid on a clean area. Bubbles support calcite. Confirm with crystal habit or texture, especially for fibrous gypsum or well-formed calcite rhombs. Do not scratch polished, valuable, or museum-quality specimens; use an existing broken edge where permitted. Use dilute acid only with eye protection and proper procedures. Gypsum powder may dissolve without bubbling, which is not effervescence. Avoid taste tests and minimize dust from soft, powdery samples.

  • Quartz vs Calcite — Tell quartz from calcite with hardness, cleavage, fracture, dilute-acid response, and crystal habit using a safe field workflow.
  • Calcite vs Dolomite — Compare calcite and dolomite using cool dilute acid, powdered-surface response, hardness, cleavage, and mixed-carbonate cautions.
  • Fluorite vs Calcite — Distinguish fluorite from calcite with four-way versus three-way cleavage, Mohs hardness, dilute-acid response, and UV limitations.
  • Halite vs Calcite — Tell halite from calcite safely using cubic versus rhombohedral cleavage, hardness, water solubility, and dilute-acid response—without tasting.
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