From California Historical Landmarks:
This tablet commemorates the discovery of gold-bearing quartz and the beginning of quartz mining in California. The discovery was made on Gold Hill by George Knight in October 1850. The occurrence of gold-bearing quartz was undoubtedly noted here and elsewhere about the same time or even earlier, but this discovery created the great excitement that started the development of quartz mining into a great industry. The Gold Hill Mine is credited with a total production of $4,000,000 between 1850 and 1857.