Contrary to the placers where gold could be found in former or actual river beds in pure, granulated form, a lot of California gold was encapsulated in rocks. These rocks had to be mined and then mechanically crushed. Afterwards, amalgamization with mercury withdrew the gold from the stone dust. Quartz mining thus required the highest level of technique and financial investment. Therefore, quartz mining became common only from 1850 onwards and usually involved large mining companies who employed wage laborers.