Calcite

Calcite is one of the most common and most versatile minerals on Earth. It's the primary mineral in limestone, marble, and chalk. It forms stalactites and stalagmites in caves. It makes up coral reefs and seashells. It precipitates from hot springs and volcanic vents. It fills fractures in virtually every rock type. No other mineral forms in as many different environments or takes as many different shapes.

Calcite crystallizes in over 800 known crystal forms - more than any other mineral. The most common are rhombohedrons (tilted cubes), scalenohedrons (pointed, dog-tooth shapes), and prismatic crystals. This extraordinary polymorphism makes calcite the most visually diverse mineral in any collection.

Iceland spar - perfectly transparent calcite - demonstrates one of the most dramatic optical properties in mineralogy: double refraction. Place a piece of Iceland spar over text and you see doubled letters. This occurs because calcite splits light into two polarized beams traveling at different speeds through the crystal. Viking navigators may have used Iceland spar as a polarizing filter to locate the sun on overcast days.

Calcite is identified by three properties: hardness 3 (scratched by a copper coin), perfect rhombohedral cleavage (breaks into tilted parallelogram shapes), and vigorous effervescence in dilute hydrochloric acid (it fizzes dramatically). These three tests together are definitive.

Distinguish from quartz (much harder at 7, no acid reaction), aragonite (same chemistry but different crystal system), and dolomite (slower acid reaction, only fizzes in hot acid or when powdered). Calcite's rhombohedral cleavage - visible in any broken piece - is one of the most diagnostic features of any mineral.