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.