Calcite: soft, reactive, and sometimes double vision

Calcite is a 3 on the Mohs scale, soft enough to scratch with a copper penny, and it fizzes readily with dilute acid, the classic and most reliable field test for the mineral. Calcite occurs in an enormous range of colors and crystal habits, from simple rhombohedral crystals to elaborate scalenohedral points. A genuinely striking optical property shows up in very clear specimens, sometimes called Iceland spar, which display double refraction, meaning an image viewed through the crystal appears doubled, a real and easily demonstrated physical phenomenon caused by the way the crystal structure splits light into two separate rays.