Calcite's strong birefringence (δ = 0.154–0.174) was first described by Danish scientist Rasmus Bartholin in 1669 using transparent Iceland spar.
The largest known calcite crystal, from Iceland, weighs about 250 tons and measures roughly 7 m × 7 m × 2 m.
In World War II high-grade optical calcite was used for gun sights, and Iceland spar has been proposed as the 'sunstone' Vikings may have used to locate the sun under overcast skies.