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Fluorite vs Calcite: Cleavage, Hardness and Acid

Fluorite and calcite can both be colourless, white, green, purple, or glassy and commonly occur together in veins. Fluorite has Mohs hardness 4 and four cleavage directions that form octahedra. Calcite has hardness 3, three rhombohedral cleavage directions, and reacts readily with cool dilute acid. Count cleavage geometry and then use acid if appropriate. Fluorite cleaves in four directions to produce octahedral fragments and does not effervesce; calcite cleaves in three directions to produce rhombs and bubbles readily in cool dilute acid. Rotate the specimen and trace repeated cleavage planes. Four-direction octahedral geometry favours fluorite; three-direction rhombs favour calcite. Use a steel point carefully. Calcite scratches more readily than fluorite, though both are softer than common glass. If trained and equipped, use one drop of cool dilute acid. Immediate effervescence supports calcite. Use UV response only as supporting evidence because not every fluorite specimen fluoresces and other minerals can fluoresce. Do not use colour or fluorescence alone; both minerals vary widely and may occur together. Use only a proper enclosed UV lamp with eye and skin protection, and follow dilute-acid procedures. Do not scratch or acid-test collectible crystal faces when a broken edge or loose fragment is unavailable.

  • 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.
  • Gypsum vs Calcite — Tell white or clear gypsum from calcite with fingernail hardness, cleavage, dilute-acid response, and non-destructive checks.
  • 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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