Complex sentence: Late in the 18th century, copper miners in Kazakhstan thought they found an emerald deposit of their dreams, they found fantastic cavities in quartz veins in a limestone, filled with thousands of lustrous emerald-green transparent crystals.
Split into simpler sentences: Late in the 18th century, copper miners in Kazakhstan thought they found an emerald deposit of their dreams. They found fantastic cavities in quartz veins in a limestone, filled with thousands of lustrous emerald-green transparent crystals.