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Quartz vs Calcite: The Fastest Field Tests

Quartz and calcite can both be white, colourless, glassy, crystalline, or massive. Colour is therefore a weak separator. Quartz is much harder, lacks cleavage, and does not normally effervesce in dilute acid. Calcite is softer, has three directions of rhombohedral cleavage, and reacts readily with dilute acid. Try hardness first: quartz scratches common glass, while calcite is easily scratched by a steel nail. Calcite also shows rhombohedral cleavage and effervesces with dilute acid; quartz commonly shows conchoidal fracture and no reaction. Use a sharp, fresh edge on expendable glass and wipe away any powder. A permanent groove strongly supports quartz. Try a steel nail on the specimen. Calcite scratches readily; quartz normally resists. Rotate the sample under light and look for repeated flat cleavage planes. Calcite cleavage forms rhombohedral geometry; quartz lacks cleavage. If trained and equipped, place one drop of cool dilute acid on a clean surface. Calcite commonly fizzes; quartz does not. Confirm with crystal habit and geologic context rather than relying on a single test. Never use a phone screen, lens, vehicle window, or valuable surface for a hardness test. Use acid only with eye protection, appropriate training, ventilation, and site procedures. Household vinegar may react more slowly than dilute hydrochloric acid. Quartz grains attached to a softer specimen can create a false glass scratch. Test a clean, representative part of the unknown.