(Left) Two water-worn gold nuggets from Tuolumne County. They are typical of larger nuggets found by the early California gold rush placer miners (each ~1.6 x 1.1 x 0.3 cm). (Right) Crystalline gold specimen from the California Mother Lode, probably from Tuolumne County (5.3 x 2.7 x 2.4 cm).
Mother lode is a principal vein or zone of gold or silver ore. The term is also used colloquially to refer to the real or imaginary origin of something valuable or in great abundance.