Calcite (optical, aka Viking sunstone) - Mexico

According to tradition, Vikings could locate the sun, on a completely overcast day,  by looking through optical-quality, double-refractive Calcite.  This allowed them to navigate by the sun.

These are cleaved natural crystals, which vary widely in shape/size, but are generally about 1 cubic-inch overall, and generally weigh about 20gms.

Double-Refractive Calcite would later form the basis of most early scientific experiments on light.

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